The present study was approved by the University of Konstanz’s Institutional Review Board and adhered to the guidelines of the German Psychological Society and the Declaration of Helsinki.
Design and procedures
The study was conducted over the course of four weeks. ‘Regular’ sugar shakers (default) were used in the first and third week, while in the second and fourth week they were functionally modified to release a smaller amount of sugar in each pour (nudge). ‘Regular’ sugar shakers were introduced as part of the study to allow an inconspicuous modification of the sugar shakers between study weeks. In addition, at the end of the study a subset of the customers who had added sugar to their drinks was surveyed to evaluate the study (questionnaire implemented via Qualtrics (version 1.6.4.)). All participants gave written informed consent and received a coffee voucher for participating.
Materials and measures
Setting
The study was conducted at the take-away café of the University of Konstanz (Germany) which is centrally located in the main foyer of the university close to the main entrance. The take-away café is the main café of the university and represents a central meeting point for the about 10,000 students and 2300 staff of the university. Besides hot beverages, the café offers a small selection of packaged chilled drinks and small snacks (e.g. pretzel, muffin, croissant, apple, candy bar) for take-away. The café consists of a cashier’s counter for ordering and collecting drinks without any seating available. Sugar shakers are placed on a table opposite the bar together with additional milk, spoons and napkins, which customers are free to use.
Intervention material
A common sugar shaker (Fackelmann, capacity 280 ml) was used as the default option. For the nudging condition, the diagonal of the hole was reduced from 1.0 cm to 0.4 cm by inserting a small metal pipe in the spout (see Fig. 1). To determine the amount of sugar dispensed per pour, a sugar shaker was filled with 200 g of regular granulated white sugar for each of the two conditions (default/nudge) and the average amount dispensed was measured over 25 pours. The procedure was repeated four times for each condition. The modification reduced the average amount of sugar in each pour by 47% from 2.96 g (SD = 0.08; default condition) to 1.58 g (SD = 0.07; nudge condition), which reduced the average calories per pour from 11.5 kcal to 6.1 kcal.
Sugar use per hot beverage
As a close proxy for sugar intake, the amount of used sugar was assessed. Each day, before the café opened, the shakers were filled with 200 g of standard granulated white sugar and in the evening, after the café closed, sugar shakers were weighted to assess the remaining sugar. For weighing, a standard kitchen scale measuring to an accuracy of 1 g was used. To determine sugar intake per hot beverage (in g), the amount of used sugar was divided by the number of hot beverages registered as sold in the cashier system. Hot beverages included the café’s typical selection of drinks such as various coffee products (i.e., coffee, café crème, cappuccino, latte macchiato, espresso, milk coffee) and hot chocolate, chai latte, and tea.
Customer questionnaire
Customers used a 7-point Likert scale to evaluate the intervention on four different dimensions (effective, helpful, annoying, adequate), following Kroese et al. [12] and assessed the strength of perceived influence on personal sweetening behavior, i.e., perceived lack of agency (1-'not at all' to 7-'strongly'). In addition, demographics, i.e., age, gender, self-reported height and weight as well as professional status, were also assessed.
Analytical procedure
To compare default and nudging condition a t-test for independent samples was conducted. The effect size was calculated following Rasch, Friese, Hofmann, and Naumann [13]. All analyses were conducted with IBM SPSS Statistics (version 25).