Statistical analyses
SECONDARY
We followed an analysis plan pre-registered on Open Science Framework (EF single scores were standardised within the 66 meal components and the 30 ready-to-eat meals shelves because the scores of food products composing each shelf had different distributions (average environmental impact for the meal components shelf: 0.61 ± 0.86, ready-to-eat meals shelf: 0.54 ± 0.39). The standardisation allowed the comparison of the two tasks in the same statistical model. To examine the effect of the environmental label on the environmental impact of food choices, the primary analysis consisted in a mixed model that tested the effects of labelling (categorical variable: environmental label or no label), food choice task (categorical variable: choice of a composed meal or a ready-to-eat meal), labelling*food choice task interaction on standardised EF single scores (everyday meal scenario only), with random effect of participants to account for correlation between repeated measures. Parameters estimates were calculated after removing non-significant interaction.In sensitivity analyses, we tested whether the results from the primary mixed model differed: 1/ after excluding aim-guessers, 2/ after excluding outliers on the primary outcome, 3/ after adjusting for age, gender, highest education level and BMI and 4/ after replacing standardised EF single scores by standardised GHGEs, ozone depletion and particulate matter scores.To examine existing knowledge and additional information provided by the environmental label, the main effect of food choice scenario (categorical variable: everyday meal scenario or environmentally friendly meal scenario), as well as labelling*scenario and scenario*food choice task interactions, were added to the primary mixed model presented above. Pairwise post-hoc comparisons were conducted to compare the environmental impact of food choice between the everyday meal and environmentally friendly meal scenario in participants with no label (i.e., highlighting existing knowledge) and to compare the environmental impact of food choice between no label and environmental label condition in the environmentally friendly meal scenario (i.e., highlighting additional information provided by the label). Parameters estimates were calculated after removing non-significant interactions.As secondary analyses we examined the effect of the environmental label on secondary outcomes with mixed models testing the effect of labelling, food choice task and labelling*food choice task interaction on nutritional quality, price per kcal, liking and familiarity (everyday meal scenario only), with random effect of participants to account for correlation between repeated measures.As exploratory analyses, Chi-squared tests were performed to test whether labelling condition was associated to meal types (beef or lamb-based meals, poultry or pork-based meals, fish-based meals, dairy or egg-based meals and plant-based meals) in the everyday meal scenario for Task 1 (composed meal) and Task 2 (ready-to-eat meal) separately.