Tackling the challenges of food waste diary studies - Testing strategies with Finnish data.

This study reports food waste diary results in Finland from 2019 (296 households) and 2022 (372 households) using an online food waste diary Kotihukka. According to the results, Finnish households waste 15.5-16.9 kg (SD 16.7-17.5 kg) of edible food per person per year. The results are an underestimate, but diary method is still a suitable method for collecting detailed and household specific food waste data and monitoring the relative difference in waste volumes. Furthermore, to evaluate the results, it is important to concentrate on 1) sample requirements for repeated measures, 2) needed resources, and 3) methodological restrictions. First, food waste diary studies often lack evidence-based discussion on the adequacy of data to monitor the direction of food waste. This study demonstrated that the sample sizes and length of the observation period (14 days) would be sufficient to detect a population-level decrease of 20-25 % in the amount of food waste during the next decade. Secondly, as bigger sample sizes require more resources, the necessity of kitchen scales was systematically studied for the first time. This study found evidence that households can report food waste as reliably without the assistance of kitchen scales as with kitchen scales, and therefore it is possible to cut costs and improve scalability without causing measurement bias. Third, since often most motivated participants attend the diary studies, this study also provides strong evidence that a self-selection bias can be reduced with monetary compensation: the household dropout rate decreased from 29 % to 7 %.