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A randomized controlled trial of nutrition education to promote farmers’ market fruit and vegetable purchases and consumption among women enrolled in the Special Supplemental Nutrition Program for Women, Infants and Children (WIC): rationale and design of the WIC Fresh Start program

  • Jennifer Di Noia1Email author,
  • Dorothy Monica2,
  • Karen Weber Cullen3 and
  • Alla Sikorskii4
BMC Nutrition20151:33

https://doi.org/10.1186/s40795-015-0032-8

Received: 10 October 2015

Accepted: 13 November 2015

Published: 17 November 2015

Abstract

Background

This report describes the protocol guiding the design and evaluation of a theory-driven, web-based lesson to promote farmers’ market fruit and vegetable (FV) purchases and consumption among women enrolled in the Special Supplemental Nutrition Program for Women, Infants and Children (WIC). Designed to leverage vouchers provided to WIC participants for FV purchases through the Farmers’ Market Nutrition Program (FMNP) and monthly cash value vouchers (CVVs) redeemable at farmers’ markets, the lesson is conceptually grounded in formative research on knowledge, attitudes and skills influencing farmers’ market FV purchases and consumption and theoretical understanding of approaches for modifying them.

Methods/Design

The setting is a large WIC agency serving three New Jersey counties. Separate samples of women were recruited to participate in 1) focus groups for guiding lesson content development (N = 56) and pretesting the resulting content (N = 52), 2) cognitive testing to assess the clarity and interpretability of items and response formats in measures of knowledge, attitudes and skills developed for the study (N = 15), 3) one-on-one sessions to assess reactions to initial versions of video segments developed for the lesson (N = 20), and 4) the outcome evaluation (N = 744). Stratified based on FMNP voucher receipt, participants are randomized to receive the lesson or existing online health education. Outcome measures (administered orally to reduce literacy demands of the response task) are completed at pretest (immediately before the lesson), posttest (2 weeks after the lesson), and 3 and 6 months after posttesting. Short- and long-term lesson effects on FV intake, FMNP voucher redemption and the redemption of CVVs at farmers’ markets will be evaluated. Evidence for mediation by knowledge, attitudes and skills of lesson effects on FV intake and voucher redemption, dose-response relationships, and user satisfaction with the lesson also will be examined.

Discussion

Theory-driven, web-based nutrition education expressly designed for WIC to promote farmers’ market use is lacking. The WIC Fresh Start program addresses the paucity of programs of this type. Findings will advance understanding of effective approaches for promoting farmers’ market FV purchases and consumption among WIC participants.

Trial Registration

NCT02565706 (registered September 30, 2015)

Keywords

Randomized controlled trial Farmers’ market Farmers’ market nutrition program Cash value voucher Fruits Vegetables Income WIC program Internet

Background

Fruit and vegetable (FV) consumption is associated with decreased incidence of and mortality from cardiovascular diseases, cancer, diabetes and obesity [13]; yet, less than 5 % of U.S. adults consume recommended amounts of FVs [4]. Low income is a risk factor for low FV intake [57], highlighting the need for dietary intervention in low-income groups. Low-income women of childbearing age may benefit disproportionately from intervention as optimal dietary intake influences nutritional status and has implications for neonatal and infant development [8].

The Special Supplemental Nutrition Program for Women, Infants and Children (WIC) is designed to safeguard the health of low-income pregnant and postpartum women, infants and children up to 5 years who are at nutrition risk by providing healthy foods to supplement diets (via WIC food packages), nutrition education, and counseling and referrals to other health, welfare and social services [9]. The program is unique in that it provides nutrition education to all participants and to caregivers of infant and child participants [10]. WIC serves over 9 million mothers and young children annually, over 1.5 million pregnant and breastfeeding mothers, more than half of America’s infants and 25 % of its children aged 1–5 years [11], providing an unparalleled opportunity to promote FV consumption through effective nutrition education.

In 1992, Congress established the WIC Farmers’ Market Nutrition Program (FMNP) to provide fresh, unprepared, locally grown FVs to WIC participants and to raise awareness, use of, and sales at farmers’ markets and roadside stands [12]. The program provides WIC participants vouchers redeemable for 10–$30 of produce from farmers authorized to accept the vouchers during the farmers’ market season (typically June through November). [12]. Previous research has shown that WIC participants who receive FMNP vouchers have higher vegetable intake and higher intake of FVs combined relative to those who do not [1316]. Farmers’ market use is greater among WIC participants who previously redeemed FMNP vouchers as compared to those who did not receive or redeem them [17]. Relative to vouchers alone, stronger FMNP effects on FV intake are found when FMNP vouchers are supplemented with nutrition education [18].

In 2009, changes to the WIC food package included the addition of a cash value voucher (CVV) for FV purchases [19]. CVVs are valued at $10 for women and $6 for children [19]. WIC state agencies have the option to approve CVVs for use at farmers’ markets [19]; roughly one-fourth (23 %) of agencies do so [20]. Whereas FMNP vouchers are issued on a seasonal basis and not every family is guaranteed to receive them due to funding constraints, CVVs are issued monthly to all WIC participants [21]. As such, CVVs afford WIC participants ongoing opportunities to purchase FVs at farmers’ markets in states approving their use at these venues.

Despite the promise of FMNP vouchers and CVVs for improving access to fresh FVs among WIC participants, the FMNP voucher redemption rate is low (59 % nationwide in 2014) [22]. Because the USDA tracks CVV use in the aggregate regardless of where vouchers are redeemed, national-level data on CVV redemption at farmers’ markets are lacking [20]. Among states reporting this information, the rate of redemption is less than 1 % [20]. Interventions to improve advertising, outreach and coordination among key stakeholders (e.g., farmers, government representatives and farmers’ market managers), show promise for improving CVV redemption at farmers’ markets [20]. Far less is known, however, about the utility of nutrition education for accomplishing this goal. Evidence of improved FMNP voucher redemption when FMNP vouchers are supplemented with nutrition education seems to suggest the promise of nutrition education for also increasing the redemption of CVVs at farmers’ markets [18]. Yet, nutrition education expressly designed for WIC to encourage farmers’ market use is lacking. This report describes the protocol guiding the design and evaluation of the WIC Fresh Start program, a theory-driven, web-based nutrition education lesson to promote farmers’ market FV purchases and consumption among WIC participants. The project is a partnership between a university-based researcher and New Jersey state and local WIC agency representatives. New Jersey is among the states authorizing the redemption of CVVs at farmers’ markets.

Research objectives

  1. 1.

    Develop a theory-driven, web-based nutrition education lesson to promote farmers’ market FV purchases and consumption among WIC participants.

     
  2. 2.

    Evaluate lesson effects on FV intake, FMNP voucher redemption and redemption of CVVs at farmers’ markets immediately following and 3 and 6 months after the lesson in a randomized four-arm design (lesson, lesson + FMNP vouchers, existing online health education, and existing online health education + FMNP vouchers).

     

Study hypotheses

Primary (outcome evaluation)

  1. 1.

    Women in the lesson + FMNP vouchers condition will have higher FV intake at posttest and follow-up measurements relative to women in the three other conditions.

     
  2. 2.

    Women who receive the lesson will have higher FV intake at posttest and follow-up measurements relative to women who receive existing online health education.

     
  3. 3.

    Women who receive FMNP vouchers will have higher FV intake at posttest and follow-up measurements relative to women who do not receive FMNP vouchers.

     
  4. 4.

    At posttest and follow-up measurements, the redemption of CVVs at farmers’ markets will be higher among women who receive the lesson relative to women who receive existing online health education.

     
  5. 5.

    FMNP voucher recipients who receive the lesson will have higher voucher redemption at posttest and follow-up measurements relative to those who receive existing online health education.

     

Secondary (process evaluation)

  1. 6.

    Lesson effects on FV intake and voucher redemption will be mediated by improvements in targeted knowledge, attitudes, and skills.

     
  2. 7.

    Receipt of more of the lesson will be associated with more favorable outcomes.

     
  3. 8.

    User satisfaction will be higher among women who receive the lesson relative to women who receive existing online health education.

     

Methods/design

Trial design

The evaluation employs a four-arm, longitudinal randomized design. Stratified based on FMNP voucher receipt, participants are orally administered a pretest and randomized to receive the lesson or existing online health education at the collaborating WIC agency. Two weeks after the lesson, participants are contacted by telephone to complete the posttest. Telephone-administered follow-up assessments are conducted 3 and 6 months after posttesting.

Trial status

At the time of manuscript submission, recruitment has been completed. Data collection has started and is ongoing.

Setting

The setting is a large WIC agency serving 21,500 participants across three New Jersey counties monthly. All study data are being collected at this location.

Participants and procedure

Pregnant and postpartum WIC participants and female caregivers of infant/child participants (i.e., those eligible to receive WIC nutrition education) with no known restrictions on food intake who are not high-risk are eligible for the study. Trained research assistants (RAs) screened women for eligibility when presenting for services. Eligible women received oral and written descriptions of the study and provided informed written consent prior to their study involvement. The study was approved by the William Paterson University Institutional Review Board for Human Subject Research (2014–368) and registered with ClinicalTrials.gov (NCT02565706). It is being conducted and will be reported in accordance with CONSORT guidelines [23]. Recruitment for the lesson development phase of the project (research objective 1) began in July 2014. Enrollment for the outcome evaluation (research objective 2) began in June 2015.

Sample size and power

The primary outcome is FV intake. Two dimensions are assessed: frequency of intake (times per day FVs are consumed) and quantity of intake (cups per day of FVs consumed). Preliminary data on FV servings per day were available and were used to estimate the standardized effect size needed for power calculations. Systematic reviews of FV interventions reveal post-intervention between-group differences in FV intake of .42–2.5 servings/day in minority and low-income adults [24, 25]. Conservatively, the study is powered to detect a .60 serving/day difference. Analyses for testing hypotheses 1–3 include covariance adjustment for pretest FV consumption. Assuming a population standard deviation of 1.5 servings and .20 correlation between pretest and posttest measures of intake (based on research in WIC samples) [13, 26], 380 women (95 per arm) are needed to detect a difference of this magnitude in pairwise posttest comparisons by arm (standardized effect size = .41). When follow-up measurements are considered, the error variance is further reduced due to 3 repeated measures and pretest covariance adjustment. The sample is sufficiently large to detect an effect size of .26 or greater in longitudinal analyses. Allowing for 40 % attrition from pretest to final follow-up (based on rates of attrition in other WIC samples) [18, 27], the sample size was inflated a priori by 1.66 (1/.60). This inflation yielded a sample size of 630 at time of recruitment. As sufficient numbers of women in the FMNP stratum were not enrolled during the recruitment phase of the study, the phase was extended by 2 weeks. In total, 744 women were enrolled.

Randomization sequence generation and allocation concealment

One of the investigators generated a binary randomization sequence using SAS© (version 9.4, 2013, Cary NC). Blinded to group assignment, RAs recruited participants and recorded their names on a form. Information on whether the participant was allocated to receive the lesson or existing online health education was preprinted in a column adjacent to the space provided for recording names and was concealed by a cover page. The recruiting RA assigned the participant to a second RA who met with the participant in a 1:1 session during which the consent form was reviewed/completed, the pretest was administered, and the participant watched an online lesson. The enrolling RA informed the second RA of whether the participant would receive the lesson or choose from among existing online lessons at the start of the 1:1 session. RAs are unaware of which lesson is being evaluated in the study.

Intervention

Overview

The intervention is an online lesson to promote farmers’ market FV purchases and consumption. Women in the control group complete any of seven existing online lessons (lessons are available on breastfeeding, being active, fruits and vegetables, calcium, cholesterol, oral health and iron). The lesson comprises three modules, each consisting of 1) behavior change content presented through a video segment featuring WIC participants, and 2) an interactive activity to build targeted knowledge, attitudes, and skills. Existing online lessons consist of an introductory segment presented with online text and graphics. After reading this material, women have the option to complete one of four lesson activities. The activities provide opportunities for women to read further on the topic and are designed to reinforce key points of the lesson.

Preliminary work

Formative research with state and local WIC agency representatives (N = 5) and participants served by the collaborating WIC agency (N = 54) identified influences on farmers’ market FV purchases and consumption that are the focus of the lesson, i.e., lack of knowledge of the FMNP and WIC-authorized farmers’ markets (locations, hours of operation and transportation options to markets), negative attitudes towards farmers’ market FVs (including farmers’ market FV food safety concerns), limited awareness of locally grown, seasonal FVs (items that are in season and selection, storage and parts eaten of the items), limited asking skills (extent of asking farmers about their participation in the FMNP and their produce), limited FV food safety and farmers’ market FV preparation skills, and low positive outcome expectations for consuming locally grown FVs.

Given the advantages Internet interventions afford (e.g., scalability or accessibility by many people, simultaneously and repeatedly; uniformity of delivery; the ability to provide customized, engaging and interactive content and low delivery costs) [28, 29] and interest in designing a resource that would fit well within WIC clinic operations, the lesson is delivered online. In 2009, the New Jersey WIC program launched an interactive website offering nutrition and health lessons [30]. In 2011, computer kiosks were placed in all local agency administrative offices to enable WIC participants to complete lessons while in the WIC clinic [30]. The expansion of topics is one component of the state’s plan for enhancing revisit rates to the website [30].

Women from the collaborating WIC agency were primarily African American (45 %) and Hispanic (44 %), with 63 % of Hispanics reporting Spanish as the primary language spoken at home. In light of the high proportion of participants speaking Spanish as their primary language, English- and Spanish-language versions of the lesson were developed. Women had less formal education for their age group than the U.S. population average (67 % had earned a high school diploma; 19 % had not completed high school) [31]; as such, they are likely to experience low health literacy [32]. Content delivery is therefore through short video segments and audio output to maximize accessibility for learners with low literacy and numeracy skills [33]. Because lesson activities require user input, to maximize comprehension, audio instruction on the completion of activities is provided, questions and responses are read aloud as they appear on the screen and viewers can hear them again by clicking on audio icons adjacent to on-screen text. To enhance community ownership of and the credibility and relevance of messages, WIC participants were engaged as full partners in the development of the lesson (as described below) and the delivery of content (videos feature participants). Owing to the strong oral tradition of the primarily African American and Hispanic women for whom the lesson is developed, key messages are conveyed through participant experiential narratives or firsthand stories [3436].

Six versions of the lesson were planned (one for each month of the farmers’ market season featuring locally grown, seasonal items). This will allow women presenting for nutrition education in a particular month to receive information on FVs they are likely to find at farmers’ markets at that time of year. For the current study, the lesson for the month of July was developed.

In a serial format or single narrative spanning the three modules, WIC participants featured in the lesson discuss challenges that prevented them from purchasing and consuming farmers’ market FVs (exposition and beginning of conflict), events serving as a catalyst for change (complication and climax), ways they overcame the challenges and strategies (education and skills training) for helping viewers do the same (resolution) [37]. Lesson modules, goals (targeted outcome changes), content and activities are summarized below.

Lesson modules

Farm fresh

Goal: To increase knowledge of the FMNP and WIC-authorized farmers’ markets and promote favorable attitudes towards farmers’ market FVs. Content: Viewers are introduced to three WIC participants shown waiting for a nutrition class at the collaborating WIC agency. Each shares “her story” (reasons she did not shop at farmers’ markets, something that happened to change her mind about this, and lessons learned that she will share with viewers). Thereafter, the first participant discusses common misperceptions about farmers’ market FVs and provides corrective feedback based on her experiences. Activity: Viewers enter their zip code into a Farmers’ Market Locator Tool and are provided with information on the three WIC-authorized markets nearest to their home. They have the option to search in another zip code area and can email themselves information about the different markets.

Market smarts

Goal: To promote positive outcome expectations for consuming locally grown FVs and improve farmers’ market FV knowledge and asking skills. Content: Featuring the second WIC participant filmed at a local farmers’ market, the participant discusses FVs that are in season, reasons to choose locally grown seasonal items, and featured items (selection, storage and parts eaten of three locally grown, seasonal items). Throughout the discussion, the participant emphasizes the importance of getting to know local farmers. A local farmer also is featured. The farmer discusses his motivation for selling his produce at the market (to educate consumers about different FVs that are locally available), his appreciation for having customers ask about the FVs he grows, and his hope that WIC participants will take advantage of the fresh FVs local farmers have to offer, noting that he and other farmers are glad to work with WIC participants and realize how important it is for women and children to have high quality FVs. Activity: Market Smarts Quiz (viewers respond to true/false items about featured items).

In the kitchen

Goal: To improve FV food safety and preparation skills. Content: Featuring the third WIC participant shown in a kitchen, the participant demonstrates the safe handling of FVs, including tips specific to the three items featured in Market Smarts. Video demonstrations of recipes for preparing the items also are shown. Only recipes that can be made with other items in the WIC food package are presented. Activity: Viewers rehearse steps for making a FV recipe by placing, in the correct order, picture cards depicting the different recipe steps.

Immediately after completing the lesson, women receive a packet of handouts highlighting information presented in the lesson. One, two, and three months after the lesson they receive a follow-up email that links to a video featuring WIC participants shown in the lesson. Participants “check-in” with viewers to ask if they have been to a farmers’ market, reinforce key messages presented in the lesson and demonstrate a new recipe for preparing a featured FV item.

Conceptual model

The lesson is grounded in formative research on knowledge, attitudes and skills influencing farmers’ market FV purchases and consumption and theoretical understanding of approaches to modifying them. Social Cognitive Theory supports leveraging environmental FV resources (i.e., promoting FMNP voucher and CVV redemption); use of credible and relatable role models (i.e., WIC participants) and feedback to foster observational learning and outcome expectations; and education, experiential activities (e.g., web-mediated skills-building exercises) and delivery modalities that engage and sustain learners’ interest to build knowledge and skills [3740]. Culture-centric approaches to health promotion program development emphasize the use of narratives or stories to facilitate behavior change [36, 37]. Narratives among audience members about experiences with a focal issue may ground programs so that they are more culturally meaningful [36]. Moreover, behavior change can be enhanced when stories address behavior-change issues and the lesson communicated in the story promotes health behavior change [37]. The Transportation-Imagery Model attributes narrative persuasion to “transportation,” becoming highly absorbed with a story. Transportation makes the story seem more like actual experience; transported individuals are less likely to counter-argue and to believe story propositions and to adopt the beliefs of characters with whom they identify [41, 42].

Development

Lesson development was informed by a community-based participatory research approach actively involving key stakeholders, i.e., state and local WIC agency representatives and WIC participants, as full partners in all phases of the work [43, 44].

WIC advisory board

At the start of the project, an advisory board of WIC agency administrators and staff was convened to meet monthly throughout the project. The board assisted with the development of the lesson and is providing ongoing direction on the coordination of study activities. Monthly meetings document materials reviewed by the board and corresponding recommendations; problems, if any, encountered and strategies to avoid or resolve them; and timeline adherence.

Focus groups

Four focus groups per module (two in English and two in Spanish) were planned to gather feedback specific to the different modules. In total, 14 groups were held, as additional feedback was sought regarding one of the modules. Trained bilingual (English-Spanish) RAs moderated the groups. Groups consisted of 4–5 participants each (N = 56 participants total) and were 50 min in length. To obtain in-depth information from participants, three questions per group were asked (the questions differed for each module), affording each participant 3 min to respond to each question. Groups of questions (and corresponding modules) were focused on attributes of WIC participants featured in the lesson, settings in which shown, and events serving as catalysts for behavior change (Farm Fresh); perceived rewards of eating FVs, reasons to eat locally grown seasonal items, and focal FVs [participants sorted picture cards depicting locally grown seasonal items into piles of familiar and unfamiliar items and selected one item in each pile that they would like to learn more about] (Market Smarts); and reactions to a 4-min FV food safety video (what was liked and what, if anything, might be difficult to do) [45] and FV recipes (In the Kitchen). The groups were tape-recorded. Audiotapes were transcribed and analyzed using tape-based analysis [46]. Findings guided the development of a written curriculum of lesson content.

The written curriculum guided the development of storyboards for each module. Using established guidelines [47], a second round of 12 focus groups was held (N = 52 participants) to gather feedback on 1) likely effects of the module on targeted knowledge, attitudes and skills, 2) content, if any, that should be eliminated, and 3) content, if any, that should be changed to increase potential effects. Data were analyzed using the same approach as that used to analyze data from the first set of groups [46]. The written curriculum was revised based on the feedback.

Video production

Advisory board members identified English- and Spanish-speaking WIC participants to narrate lesson modules based on character attributes identified in the first round of focus groups. Videos were filmed on location in settings identified by the groups. WIC participants agreeing to narrate the modules received $50 gift cards (redeemable at local supermarkets and chain stores) for each half-day of filming. English- and Spanish-language storyboards developed for the second round of focus groups served as video scripts. RAs assisted the narrators in rehearsing the scripts.

Pretesting video segments

Participant reactions to “rough cuts” of video segments developed for the first lesson module were assessed in one-on-one sessions with RAs using the think aloud method [48]. Five participants each viewed English- and Spanish-language versions of the segments. Following a brief rehearsal of the response task, the participant previewed the focal video and verbalized reactions to it. After the video, participants were orally administered brief measures of the extent of identification with the narrator and transportation into and liking of the video (Cronbach alphas ≥ .76) [42, 49, 50]. Probes were used to explore reasons for answers given. The feedback guided edits to and/or re-filming of footage lacking quality, relevance and impact. The revised segments underwent pretesting in a second round of 1:1 sessions. Lessons learned from pretesting the segments served as a guide for filming the remaining lesson modules.

Website development

Developed in HTML, the lesson consists of a user interface with audio output, video segments, Adobe Flash activities, content managed pages (with information on local farmers’ markets for WIC administrators to edit as needed), and reporting tools (reports by WIC participant identifier of lesson modules completed). Internal testing by RAs documented the absence, if any, of required features. The documentation served as a guide for the multimedia producer to revise, beta-test, debug and finalize the lesson in preparation for the outcome study.

Measures

Although measures of adult FV-related knowledge, attitudes and skills exist, measures of farmers’ market-specific constructs are lacking and were therefore developed. Adapted from existing instruments (Cronbach alphas ≥ .70) [40, 5159], up to ten items per scale were written (some negatively worded to avoid a response set) [60]. Response options are on a yes-no, true-false, multiple choice or 7-point Likert scale format tailored to the nature of the questions. Cognitive testing with 15 participants gathered data for improving items and response formats that were unclear and/or difficult to understand [61]. The psychometric properties of the measures will be examined using data from the full administration of the instruments at pretest.

At pretest, the following sociodemographic variables are measured: date of birth, age (in years), pregnancy status, due date (pregnant women), breastfeeding status, race, ethnicity, nativity, preferred language, language(s) spoken at home, marital status, educational attainment, educational attainment of spouse or partner, number of children in the household under age 19, number of children in the household between 2 and 5 years of age, number of other adults in the household, employment status, participation in assistance programs and food security status. Because self-reports of FV intake are influenced by social approval and social desirability biases [62, 63], social desirability trait is also assessed at pretest using a validated, short form of the Marlowe-Crowne Social Desirability Scale [64].

Assessed at pretest, posttest and 3- and 6-month follow-up, outcome measures are knowledge of the FMNP and WIC-authorized farmers’ markets, attitudes towards farmers’ market FVs, awareness of locally grown, seasonal FVs, farmers’ market FV purchases (ever purchased FVs at a farmers’ market, purchased FVs at a farmers’ market in the past 2 weeks), intentions to purchase FVs at a farmers’ market in the next 2 weeks, FV food safety skills, farmers’ market asking and FV preparation skills, and positive outcome expectations for consuming locally grown FVs (measured using instruments developed for the study). The frequency and quantity of FV intake are assessed with validated instruments [65, 66]. Frequency of intake is measured using the FV module of the Behavioral Risk Factor Surveillance System, shown to have moderate validity and reliability [65, 67]. Quantity of intake is measured with a 2-item screener developed by the National Cancer Institute [66]. The screener is valid (as evidenced by moderate correlations with FV intake assessed via multiple 24-h dietary recalls) and reliable (as evidenced by moderate 2–3 week test-retest correlations) [66]. FMNP voucher and CVV farmers’ market FV purchases are assessed using voucher redemption data provided by the state WIC agency.

Process measures include lesson dose (data recorded by RAs and tracked through the website on the number of lesson modules and activities participants completed [a total of three each for those completing the lesson and one each for those completing existing online health education]), distractions, if any, experienced during lesson play (recorded by RAs), participant self-report data on existing online lessons, if any, completed prior to the study, user satisfaction with the lesson received (ratings, on a 7-point scale, of the extent of enjoyment, interest in, and likelihood of recommending the lesson to other WIC participants), measures of new information learned from the lesson and talking to family and friends about new information learned [49], whether this was the first time completing an online WIC nutrition education lesson and the perceived novelty of the lesson [49]. Among women who receive the lesson, measures of the following also are administered: what was remembered most about the lesson [49]; what was liked and disliked about the lesson and what, if anything, could be done to improve it; transportation into the video narrative [50] and identification with the characters [42]; liking and learning from lesson activities; the activity that was liked the most; the FV the participant chose to learn a recipe about and whether the participant tried the recipe at home; and whether the participant opened follow-up emails sent after the lesson, watched the videos, and tried the recipes shown and the perceived helpfulness of the information provided. Among all participants, RAs collect information on the number screened, determined eligible/ineligible and enrolled/not enrolled (during recruitment), and follow-up calls made/completed, follow-up assessments scheduled/completed and problems, if any, encountered in reaching participants.

Data collection

All self-report data are collected by RAs in 1:1 interviews with participants (interviews are conducted in person at pretest and by telephone at posttest and follow-up measurements). One month prior to 3- and 6-month follow-up assessments, RAs telephone participants to confirm their contact information and the date/time of their follow-up interview (participants are also mailed the date/time of their appointment 1 month and 2 weeks prior to the appointment). To enhance the quality of measurement, RAs were trained in a full-day session prior to data collection. Daily half-hour debriefing sessions are used to discuss problems, if, any, encountered with data collection, ways to address them and strategies to avoid their recurrence.

Data analyses

To verify the success of randomization, between-arm differences in sociodemographic variables and prior lessons received will be examined using analysis of variance or chi-square tests as appropriate. Characteristics of women who completed the study and those lost to attrition also will be compared. Variables found to differ by arm will be included as covariates in outcome analyses. Outcome variable distributions will be assessed and normalizing transformations applied as needed. If the transformations are unsuccessful, analyses will be implemented as generalized linear models with the appropriate error distributions (e.g., Gamma or Poisson).

Outcome analyses

Using an intent-to-treat approach (i.e., all women analyzed as randomized regardless of their adherence to the protocol), analysis of covariance (ANCOVA) models will be used to test hypotheses 1–3. The models will relate posttest FV intake to pretest FV intake, study arm, and covariates identified in preliminary analyses (described above). Tests of pair-wise differences in least-square (LS) means specified in hypotheses 1–3 will determine support for each hypothesis. In a parallel fashion, lesson effects on the redemption of CVVs at farmers’ markets and on FMNP voucher redemption (among women who received the vouchers [hypotheses 4–5]) will be examined. Next, hypotheses 1–5 will be tested in a longitudinal context using linear mixed-effects (LME) models with three repeated measures. Tests of pair-wise differences between LS means specified in hypotheses 1–5 will determine whether lesson effects are sustained over time.

Mediation analyses

The method of Baron and Kenny and MacKinnon [68, 69] will be used to determine evidence for mediation by knowledge, attitudes and skills (hypothesized mediators) of lesson effects on FV intake and voucher redemption (hypothesis 6). Both single- and multiple-mediator models will be tested. Using ANCOVA for posttest measurements and LME for longitudinal follow-up, lesson effects on potential mediators will be examined. Significant lesson effects that are reduced when controlling for potential mediators (evaluated using Sobel tests) [70, 71], will provide evidence that partial mediation has occurred; the percent of variation in outcomes mediated by knowledge, attitudes and skills will be determined [72]. The absence of lesson effects after controlling for the mediators will indicate that complete mediation has occurred.

Dose-response effects

Positing that receipt of more of the lesson (as measured by the number of completed modules and activities) will foster more positive outcomes, hypothesis 7 tests dose-response relationships. This hypothesis will be examined in a series of ANCOVA and LME models (using data provided by women who received the lesson) with posttest and follow-up measures of FV intake, voucher redemption, knowledge, attitudes and skills as outcomes, lesson dose as the fixed factor, and receipt of FMNP vouchers, socio-demographic variables and pretest measures of each outcome as covariates. The significance of the coefficient for the lesson dose variable will determine support for hypothesis 7. Similarly, for those who received existing online health education, the significance of the variable reflecting which of the lessons was received will be evaluated.

User satisfaction

Descriptive statistics will examine satisfaction ratings among women who received the lesson. A mean item score ≥ 5.0 (on the 7-point scale) will be considered evidence of a high degree of satisfaction. In a test of hypothesis 8, mean satisfaction ratings will be compared to determine whether satisfaction was higher among women who received the lesson relative to those who received existing online health education. Responses to open-ended items will be summarized. The feedback will be used to identify areas for further refining the lesson.

Expected outcomes

Previous work demonstrates that FMNP effects are stronger when vouchers are supplemented with nutrition education [18]. In light of this work, it is anticipated that at posttest and follow-up measurements, the highest levels of intake will be found among women who receive the lesson and FMNP vouchers (relative to women who receive the vouchers and existing online health education and those who do not receive FMNP vouchers and receive the lesson or existing online health education). Regardless of which lesson women receive, at posttest and follow-up measurements, FV intake is expected to be higher among those who receive FMNP vouchers relative to those who do not, owing to research demonstrating that by improving economic access to FVs, the provision of vouchers leads to improvements in FV intake [1316]. Regardless of FMNP voucher receipt, FV intake and redemption of CVVs at farmers’ markets also are expected to be higher among women who receive the lesson relative to those who receive existing online health education, owing to improvements in targeted knowledge, attitudes and skills. Improvements in these outcomes also are expected to result in higher voucher redemption among voucher recipients who receive the lesson relative to those who receive existing online health education.

In tests of secondary study hypotheses, it is expected that the lesson will promote positive changes in targeted knowledge, attitudes and skills and that these improvements, in turn, will lead to increases in FV intake, FMNP voucher redemption and the redemption of CVVs at farmers’ markets. Further, it is expected that receipt of more of the lesson will be associated with more favorable changes in targeted knowledge, attitudes and skills. Finally, it is anticipated that user satisfaction will be uniformly high among women who receive the lesson and will be higher in this group as compared to women who receive existing online health education owing to features of the lesson intended to capture and sustain users’ interest, i.e., delivery through a highly accessible medium, the narrative lesson format, use of credible and relatable role models to deliver key messages and inclusion of interactive elements and activities.

Discussion

The present investigation addresses low FV intake, an issue of public health concern, among WIC participants, a population at increased risk of inadequate intake, and leverages vouchers provided by WIC to improve FV access in this population. Developed through a research partnership with New Jersey state and local WIC agency representatives, the lesson was designed to fit within WIC clinic operations, fully engage WIC participants in the development and delivery of lesson content, and modify behavior using a cost neutral approach (one allowing WIC to continue to offer the lesson at no or minimal costs after the study concludes; costs are modest and encompass fees to host the online lesson). The intervention adds to the limited number of theory-driven, web-based nutrition education lessons expressly designed for WIC to promote farmers’ market FV purchases and consumption. If effective, the lesson has the potential to exert a powerful and sustained economic and behavioral impact. Methodological strengths of the study include the randomized design, use of an active control group, validated measures of both the frequency and quantity of FV intake, objective measures of FMNP voucher and CVV redemption, and repeated measurements, which will permit examination of the sustainability of lesson effects over time. Through planned dissemination efforts, the lesson will reach other WIC participants similar to those engaged in the research. Although the study sample and location limit transferability to diverse WIC audiences in other parts of the country, the lesson may nevertheless serve as a model for the development of similar resources tailored to local needs.

Abbreviations

ANCOVA: 

Analysis of covariance

CVV: 

Cash value voucher

FMNP: 

Farmers’ market nutrition program

FV: 

Fruit and vegetable

LS: 

Least square

LME: 

Linear mixed effects

RA: 

Research assistant

WIC: 

Special Supplemental Nutrition Program for Women, Infants and Children

Declarations

Acknowledgements

This project is supported with federal funds from the U.S. Department of Agriculture, Food and Nutrition Service through grant WIC NEI-12-TX to Baylor College of Medicine to Jennifer Di Noia at William Paterson University. The contents of this publication do not necessarily reflect the view or policies of the U.S. Department of Agriculture, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. government.

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Authors’ Affiliations

(1)
Department of Sociology, William Paterson University
(2)
Saint Joseph’s WIC Program
(3)
USDA/ARS Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine
(4)
Department of Statistics and Probability, Michigan State University

References

  1. Boeing H, Bechthold A, Bub A, Ellinger S, Haller D, Kroke A. Critical review: vegetable and fruit in the prevention of chronic diseases. Eur J Nutr. 2012;51:637–63.PubMed CentralView ArticlePubMedGoogle Scholar
  2. Woodside JV, Young IS, McKinley MC. Fruits and vegetables: measuring intake and encouraging increased consumption. Proc Nutr Soc. 2013;72:236–45.View ArticlePubMedGoogle Scholar
  3. Bazzano LA. The high cost of not consuming fruits and vegetables. J Am Diet Assoc. 2006;106:1364–8.View ArticlePubMedGoogle Scholar
  4. Kimmons J, Gillespie C, Seymour J, Serdula M, Blanck HM. Fruit and vegetable intake among adolescents and adults in the United States: percentage meeting individualized recommendations. Medscape J Med. 2009;11:26.PubMed CentralPubMedGoogle Scholar
  5. Darmon N, Drewnowski A. Does social class predict diet quality? Am J Clin Nutr. 2008;87:1107–17.PubMedGoogle Scholar
  6. Kirkpatrick SI, Dodd KW, Reedy J, Krebs-Smith SM. Income and race/ethnicity are associated with adherence to food-based dietary guidance among US adults and children. J Acad Nutr Diet. 2012;112:624–35.PubMed CentralView ArticlePubMedGoogle Scholar
  7. Grimm KA, Foltz JL, Blanck HM, Scanlon KS. Household income disparities in fruit and vegetable consumption by state and territory: results of the 2009 behavioral risk factor surveillance system. J Acad Nutr Diet. 2012;112:2014–21.View ArticlePubMedGoogle Scholar
  8. Dubowitz T, Smith-Warner SA, Acevedo-Garcia D, Subramanian SV, Peterson KE. Nativity and duration of time in the United States: differences in fruit and vegetable intake among low-income postpartum women. Am J Public Health. 2007;97:1787–90.PubMed CentralView ArticlePubMedGoogle Scholar
  9. US Department of Agriculture, Food and Nutrition Service. Women, infants and children (WIC): about WIC. http://www.fns.usda.gov/wic/about-wic. Accessed 26 August 2015.
  10. Oliveira O, Frazão E. The WIC program: background, trends, and economic issues, 2015 edition. http://www.ers.usda.gov/publications/eib-economic-information-bulletin/eib134.aspx. Accessed 26 August 2015.
  11. National WIC Association. The Role of WIC in reducing infant mortality. https://s3.amazonaws.com/aws.upl/nwica.org/wics-role-reducing-infant-mortality.pdf. Accessed 26 August 2015.
  12. WIC Farmers’ Market Nutrition Program fact sheet. http://www.fns.usda.gov/fmnp/wic-farmers-market-nutrition-program-fmnp. Accessed 27 August 2015.
  13. Herman DR, Harrison GG, Afifi AA, Jenks E. Effect of a targeted subsidy on intake of fruits and vegetables among low-income women in the special supplemental nutrition program for women, infants and children. Am J Public Health. 2008;98:98–105.PubMed CentralView ArticlePubMedGoogle Scholar
  14. Kropf ML, Holben DH, Holcomb Jr JP, Anderson H. Food security status and produce intake and behavior of supplemental nutrition program for women, infants, and children and Farmers’ market nutrition program participants. J Am Diet Assoc. 2007;107:1903–8.View ArticlePubMedGoogle Scholar
  15. Galfond G, Thompson J, Wise K. Evaluation of the Farmers’ market coupon demonstration project. Alexandria: US Department of Agriculture, Food and Nutrition Service; 1991.Google Scholar
  16. Wheeler AL, Chapman-Novakofski K. Farmers’ markets: costs compared with supermarkets, use among WIC clients, and relationship to fruit and vegetable intake and related psychosocial variables. J Nutr Educ Behav. 2014;46 Suppl 2:S65–70.View ArticlePubMedGoogle Scholar
  17. Racine EF, Vaughn AS, Laditka SB. Farmers’ market use among African-american women participating in the special supplemental nutrition program for women, infants, and children. J Am Diet Assoc. 2010;110:441–6.View ArticlePubMedGoogle Scholar
  18. Anderson JV, Bybee DI, Brown RM, McLean DF, Garcia EM, Breer ML, et al. 5 a day fruit and vegetable intervention improves consumption in a low-income population. J Am Diet Assoc. 2001;101:195–202.View ArticlePubMedGoogle Scholar
  19. US Department of Agriculture, Food and Nutrition Service, Office of Research and Analysis. WIC food packages policy options study. Alexandria: FNS; 2011.Google Scholar
  20. Farmers’ market coalition. Trending at the market: cash value vouchers. http://farmersmarketcoalition.org/trending-at-the-market-wic-cash-value-vouchers/. Accessed 10 September 2015.
  21. McDonnell L, Morris MN, Holland J. WIC participants’ perceived behavioral control, attitudes toward, and factors influencing behavioral intentions to redeeming cash-value vouchers at certified farmers markets. Calif J Health Promot. 2014;12(2):22–31.Google Scholar
  22. US Department of Agriculture, Food and Nutrition Service. WIC FMNP FY 2014 FNS-203 report. Alexandria: FNS; 2014.Google Scholar
  23. Bennett J. The consolidated standards of reporting trials (CONSORT): guidelines for reporting randomized trials. Nurs Res. 2005;54(2):128–32.View ArticlePubMedGoogle Scholar
  24. Pomerleau J, Lock K, Knai C, McKee M. Interventions designed to increase adult fruit and vegetable intake can be effective: a systematic review of the literature. J Nutr. 2005;135:2486–95.PubMedGoogle Scholar
  25. Thomson CA, Ravia J. A systematic review of behavioral interventions to promote intake of fruit and vegetables. J Am Diet Assoc. 2011;111:1523–35.View ArticlePubMedGoogle Scholar
  26. Odoms-Young AM, Kong A, Schiffer LA, Porter SJ, Blumstein L, Bess S, et al. Evaluating the initial impact of the revised special supplemental nutrition program for women, infants, and children (WIC) food packages on dietary intake and home food availability in African-american and Hispanic families. Public Health Nutr. 2013;17:83–93.PubMed CentralView ArticlePubMedGoogle Scholar
  27. Krummel D, Semmens E, MacBride AM, Fisher B. Lessons learned from the mother’s overweight management study in four West Virginia WIC offices. J Nutr Educ Behav. 2010;42 Suppl 3:S52–8.PubMed CentralView ArticlePubMedGoogle Scholar
  28. Portnoy DB, Scott-Shelson LAJ, Johnson BT, Carey MP. Computer-delivered interventions for health promotion and behavioral risk reduction: a meta-analysis of 75 randomized controlled trials, 1988–2007. Prev Med. 2008;47(1):3–16.PubMed CentralView ArticlePubMedGoogle Scholar
  29. Leykin Y, Thekdi SM, Shumay DM, Munoz RF, Riba M, Dunn LB. Internet interventions for improving psychological well-being in psycho-oncology: review and recommendations. Psychooncology. 2012;21(9):1016–25.PubMed CentralView ArticlePubMedGoogle Scholar
  30. New Jersey Department of Health. Special Supplemental Program for Women, Infants and Children (WIC) FFY 2014 state strategic plan. http://www.state.nj.us/health/fhs/wic/documents/2014_state_strategic_plan.pdf. Accessed 26 November 2013.
  31. US Department of Education. The condition of education, 2013. http://nces.ed.gov/pubs2013/2013037.pdf. Accessed 20 December 2013.
  32. Paasche-Orlow MK, Parker RM, Gazmararian JA, Nielsen-Bohlman LT, Rudd RR. The prevalence of limited health literacy. J Gen Intern Med. 2005;20:175–84.PubMed CentralView ArticlePubMedGoogle Scholar
  33. Cuendet S, Medhi I, Bali K, Cutrell E. VideoKheti: Making video content accessible to low-literate and novice users. http://research-srv.microsoft.com/pubs/183489/Cuendet-CHI2013-VideoKheti-Final.pdf. Accessed 20 December 2013.
  34. Di Noia J, Furst G, Park K, Byrd‐Bredbenner C. Designing culturally sensitive dietary interventions for African Americans: review and recommendations. Nutr Rev. 2013;71:224–38.View ArticlePubMedGoogle Scholar
  35. Unger JB, Cabassa LJ, Molina GB, Contreras S, Baron M. Evaluation of a fotonovela to increase depression knowledge and reduce stigma among Hispanic adults. J Immigr Minor Health. 2013;15(2):398–406.PubMed CentralView ArticlePubMedGoogle Scholar
  36. Larkey LK, Hecht M. A model of effects of narrative as culture-centric health promotion. J Health Commun. 2010;15:114–35.View ArticlePubMedGoogle Scholar
  37. Baranowski T, Buday R, Thompson DI, Baranowski J. Playing for real: video games and stories for health-related behavior change. Am J Prev Med. 2008;34:74–82.PubMed CentralView ArticlePubMedGoogle Scholar
  38. Bandura A. Social foundations of thought and action: a social cognitive theory. Englewood Cliffs: Prentice Hall; 1986.Google Scholar
  39. Gaines A, Turner LW. Improving fruit and vegetable intake among children: a review of interventions utilizing the social cognitive theory. Calif J Health Promot. 2009;7:52–66.Google Scholar
  40. Anderson ES, Winett RA, Wojcik JR. Self-regulation, self-efficacy, outcome expectations, and social support: social cognitive theory and nutrition behavior. Ann Behav Med. 2007;34:304–12.View ArticlePubMedGoogle Scholar
  41. Hinyard LJ, Kreuter MW. Using narrative communication as a tool for health behavior change: a conceptual, theoretical, and empirical overview. Health Educ Behav. 2007;34:777–92.View ArticlePubMedGoogle Scholar
  42. Murphy ST, Frank LB, Chatterjee JS, Baezconde-Garbanati L. Narrative versus non-narrative: the role of identification, transportation, and emotion in reducing health disparities. J Commun. 2013;63:116–37.View ArticleGoogle Scholar
  43. O’Fallon LR, Dearry A. Community-based participatory research as a tool to advance environmental health sciences. Environ Health Perspect. 2002;110 Suppl 2:155–9.PubMed CentralView ArticlePubMedGoogle Scholar
  44. Bogart LM, Uyeda K. Community-based participatory research: partnering with communities for effective and sustainable behavioral health interventions. Health Psychol. 2009;28(4):391–3.PubMed CentralView ArticlePubMedGoogle Scholar
  45. Food and Drug Administration. Safe handling of raw produce and fresh-squeezed juices. http://www.foodsafety.gov/keep/types/fruits/tipsfreshprodsafety.html. Accessed 23 January 2014.
  46. Krueger RA. Analyzing and reporting focus group results. Thousand Oaks: Sage; 1998.View ArticleGoogle Scholar
  47. National Cancer Institute. Making health communication programs work. Bethesda: National Cancer Institute; 2004.Google Scholar
  48. Jaspers MW. A comparison of usability methods for testing interactive health technologies: methodological aspects and empirical evidence. Int J Med Inform. 2009;78:340–53.View ArticlePubMedGoogle Scholar
  49. Kreuter MW, Holmes K, Alcaraz K, Kalesan B, Rath S, Richert M, et al. Comparing narrative and informational videos to increase mammography in low-income African American women. Patient Educ Couns. 2010;81 Suppl 1:S6–S14.PubMed CentralView ArticlePubMedGoogle Scholar
  50. Williams JH, Green MC, Kohler C, Allison JJ, Houston TK. Stories to communicate risks about tobacco: development of a brief scale to measure transportation into a video story - The ACCE Project. Health Educ J. 2011;70:184–91.View ArticleGoogle Scholar
  51. Langenberg P, Ballesteros M, Feldman R, Damron D, Anliker J, Havas S. Psychosocial factors and intervention-associated changes in those factors as correlates of change in fruit and vegetable consumption in the Maryland WIC 5 a Day promotion program. Ann Behav Med. 2000;22:307–15.View ArticlePubMedGoogle Scholar
  52. Dibsdall LA, Lambert N, Bobbin RF, Frewer LJ. Low-income consumers’ attitudes and behaviour towards access, availability and motivation to eat fruit and vegetables. Public Health Nutr. 2003;6:159–68.View ArticlePubMedGoogle Scholar
  53. Williams LK, Thornton L, Crawford D, Ball K. Perceived quality and availability of fruit and vegetables are associated with perceptions of fruit and vegetable affordability among socio-economically disadvantaged women. Public Health Nutr. 2012;15:1262–7.View ArticlePubMedGoogle Scholar
  54. Parmenter K, Wardle J. Development of a general nutrition knowledge questionnaire for adults. Eur J Clin Nutr. 1999;53:298–308.View ArticlePubMedGoogle Scholar
  55. Byrd-Bredbenner C, Maurer J, Wheatley V, Schaffner D, Bruhn C, Blalock L. Food safety self-reported behaviors and cognitions of young adults: results of a national study. J Food Prot. 2007;70:1917–26.PubMedGoogle Scholar
  56. Kuo T, Dela Cruz H, Redelings M, Smith LV, Reporte R, Simon PA, et al. Use of a self-assessment questionnaire for food safety education in the home kitchen – Los Angeles County, California, 2006–2008. MMWR Morb Mortal Wkly Rep. 2010;59:1098–101.Google Scholar
  57. Resnicow K, Jackson A, Wang T, De AK, McCarty F, Dudley WN, et al. A motivational interviewing intervention to increase fruit and vegetable intake through black churches: results of the Eat for Life trial. Am J Public Health. 2001;91:1686–93.PubMed CentralView ArticlePubMedGoogle Scholar
  58. Barton KL, Wrieden WL, Anderson AS. Validity and reliability of a short questionnaire for assessing the impact of cooking skills interventions. J Hum Nutr Diet. 2011;24:588–95.View ArticlePubMedGoogle Scholar
  59. Taylor AW, Coveney J, Ward PR, Henderson J, Meyer SB, Pilkington R, et al. Fruit and vegetable consumption: the influence of aspects associated with trust in food and safety and quality of food. Public Health Nutr. 2011;15:208–17.View ArticlePubMedGoogle Scholar
  60. Spector PE. Summated rating scale construction. Newbury Park: Sage; 1992.View ArticleGoogle Scholar
  61. Alaimo K, Olson CM, Frongillo EA. Importance of cognitive testing for survey items: an example from food security questionnaires. J Nutr Educ. 1999;31:269–75.View ArticleGoogle Scholar
  62. Miller TM, Abdel-Maksoud MF, Crane LA, Marcus AC, Byers TE. Effects of social approval bias on self-reported fruit and vegetable consumption: a randomized controlled trial. Nutr J. 2008;7:18.PubMed CentralView ArticlePubMedGoogle Scholar
  63. Hebert JR, Hurley TG, Peterson KE, Resnicow K, Thompson FE, Yaroch AL, et al. Social desirability trait influences on self-reported dietary measures among diverse participants in a multicenter multiple risk factor trial. J Nutr. 2008;138 Suppl 1:226S–34S.PubMedGoogle Scholar
  64. Strahan R, Gerbasi KC. Short, homogenous versions of the marlow-crowne social desirability scale. J Clin Psychol. 1972;28(2):191–3.View ArticleGoogle Scholar
  65. Centers for Disease Control and Prevention. Behavioral Risk Factor Surveillance System questionnaire, 2013. http://www.cdc.gov/brfss/questionnaires/pdf-ques/2013-brfss_english.pdf. Accessed 20 March 2015.
  66. Yaroch AL, Tooze J, Thompson FE, Blanck HM, Thompson OM, Colon-Ramos O, et al. Evaluation of three short dietary instruments to assess fruit and vegetable intake: the national cancer Institute’s food attitudes and behaviors (FAB) survey. J Acad Nutr Diet. 2012;112(10):1570–7.PubMed CentralView ArticlePubMedGoogle Scholar
  67. Centers for Disease Control and Prevention. Surveillance of fruit and vegetable intake using the Behavioral Risk Factor Surveillance System. http://www.cdc.gov/brfss/data_documentation/pdf/fruits_vegetables.pdf. Accessed 19 September 2015.
  68. Baron RM, Kenny DA. The moderator-mediator variable distinction in social psychological research: conceptual, strategic, and statistical considerations. J Pers Soc Psychol. 1986;51:1173–82.View ArticlePubMedGoogle Scholar
  69. MacKinnon D. Analysis of mediating variables in prevention and intervention research. NIDA Res Monogr. 1994;139:127–53.PubMedGoogle Scholar
  70. MacKinnon DP, Dwyer JH. Estimating mediated effects in prevention studies. Eval Rev. 1993;17:144–58.View ArticleGoogle Scholar
  71. Sobel ME. Asymptotic confidence intervals for indirect effects in structural equation models. Sociol Methodol. 1982;13:290–312.View ArticleGoogle Scholar
  72. Dudley WN, Benuzillo JG, Carrico MS. SPSS and SAS programming for the testing of mediation models. Nurs Res. 2004;53:59–62.View ArticlePubMedGoogle Scholar

Copyright

© Di Noia et al. 2015