Experimental design and procedure
non-alcoholic, PIS
SHOP
To evaluate the NG labels, we employed a two-arm crossover design where participants shopped once on the following two NUSMart versions in random order (see Fig. Control condition: A default NUSMart that mirrored a conventional web-grocery store in Singapore where qualifying healthier products displayed the HCS logos.NG condition: Similar to Control condition except that all beverages displayed the appropriate NG label. Because this label is new to Singaporean shoppers, prior to this shop participants were required to watch a video explaining the new label and how it can be used to improve diet quality.Two versions of NUSMart with example products: (Participants were asked to purchase weekly groceries similar to their typical grocery shopping experience, with a minimum spend of Singapore Dollar (SGD) 50 for each shop. The minimum spend was determined based on the average weekly household expenditure on food & non-alcoholic beverages reported in the 2017/18 Singapore Household Expenditure Survey and by accounting for households with only 1 or 2 members and smaller varieties on our store than those available in FairPrice, our food delivery partner. Given that the NG labels are displayed on beverages only, we also required participants to buy at least one beverage product at each shop. This ensured they were exposed to the new labels.Participants were informed that in each of the shops there was a 50% chance that they would have to purchase their order, which would be determined by spinning a “Wheel of Purchase” immediately after checkout. When the “Wheel of Purchase” indicated that the shop was selected for purchase and delivery, the corresponding grocery cart was transferred to the FairPrice webstore where the participant managed payment and delivery via his/her FairPrice account. The products were delivered to the participant’s homes by FairPrice. This approach reduced logistical challenges while also minimizing biases that come with hypothetical shopping.A power calculation revealed that 108 participants were required to detect a 0.3 standardized effect size difference in our primary outcome of weighted (by the number of servings) average grams of sugar per serving, the primary target of the NG label, assuming a power of 0.8, a significance level of 0.05, a correlation of 0.5 in purchases across two shops, and a 20% attrition rate. The standardized effect size of 0.3 was applied based on results of prior labelling studies [Interested participants were directed to a Participant Information Sheet (PIS) and consent document. The PIS form contained the information about the study procedure and requirements but did not disclose the intervention. Once participants consented, they were directed to a baseline survey collecting demographic and health information. For the latter, we asked if any of their household members had been diagnosed with diet-related health conditions.Those who completed the baseline survey were randomized by a computer program and officially enrolled in the study. Participants were then informed by email and asked to complete two grocery shops 1 week apart. Weekly reminders were sent for those who did not complete the required shops. Upon completion of their second shop, participants were asked to fill in a short post-study survey asking if they understood/used the NG label for beverage purchases and were willing to use the label if it were displayed on food products. Participants who completed both shops and the surveys received SGD 50 via digital wallet applications for their participation. All procedures were approved by the National University of Singapore Institutional Review Board (Reference Code: NUS-IRB-2021-128). The trial was registered on ClinTrials.gov (Identifier: NCT05018026, Registered on: 24th August 2021) and can be accessed online.