First, the purpose of the interview was once again explained and informed consent was obtained. Prior to the start of the interview, we asked the participants to fill out a short questionnaire on demographic characteristics, including age, sex, and highest attained educational level. Answer possibilities for educational level were based on the Dutch education system, and lower educational level was defined as primary education up to completing intermediate vocational education. This short questionnaire was used as a screener for the researcher, in order to verify that the participants had a lower educational level which was used as a proxy for SEP. As such, the recruitment of low-SEP participants was achieved in two ways: first, by recruiting in areas which had a low neighbourhood SEP-score, and second, by verifying the highest educational level attained by the participant. If the highest attained educational level was not within the definition of the low educational level, the interview was shortened. All participants were reimbursed with a gift voucher worth EUR 20, including the participants (n = 2) who did not have a lower educational level.

We conducted semi-structured interviews that lasted on average 25 min, following a pre-defined topic list (Supplementary File S1). The topic list was used in a flexible manner by the researcher, based on the issues that were mentioned by the participants. The first part of the interview was focused on the everyday life of participants, in order to get a feeling of the relevant wider contextual factors that shape dietary choices (e.g., employment, living situation) and in order for the interviewee to feel at ease. In the second part of the interview, the participants were asked how they valued food, and what they perceived to determine their food choices. First, the participants were given the opportunity to answer freely and in their own words, and if deemed appropriate, the researcher probed by suggesting other potential determinants of food choice. In the third part of the interview, we asked under which circumstances it was easy or difficult to eat healthy for the participant. If appropriate, the researcher referred back to what was mentioned in the first and second part of the interview in order to probe further.

In the last part of the interview, the researcher showed various photos of nudges that could be applied in supermarkets to promote healthier food choices, and briefly explained them (Supplementary File S2). Instead of using the term of nudging, we introduced the photos as strategies that supermarkets could implement in order to assist customers in making more healthy food choices. Thereafter, we explored the relative judgement of the nudges by asking participants to choose the photo that appealed to them the most, and we also asked them to explain why. Vice versa, we asked participants to choose the photo which appealed to them the least, and explain why this was the case.

The study was conducted in accordance with the Declaration of Helsinki and was approved by the ethics committee of the University Medical Center Utrecht. Reporting of this qualitative study follows the guidelines set out by the Consolidated criteria for reporting qualitative research (COREQ) checklist [22].

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