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Diaries have been used to obtain national and subnational estimates of household food waste (HHFW) in several countries. Furthermore, diaries have been proposed as a method for tracking progress towards goals that include HHFW reduction. However, multiple studies have suggested diaries substantially underestimate HHFW. This paper collates and analyses data from five studies in which diary estimates of HHFW can be directly compared to other, more accurate estimates from waste compositional analysis (WCA). This analysis finds that all diary estimates for HHFW were lower than the corresponding WCA estimates, with the degree of underestimation ranging from 7% to 40%. Four main factors are likely to contribute to this underestimation: behavioural reactivity (people wasting less during the diary period), misreporting (not all items discarded being recorded), measurement bias (not all items are weighed) and self-selection bias (those completing a diary being different from the wider population). The study concludes that a) diaries are useful for obtaining approximate estimates of HHFW and detailed information on what, why, and where food is discarded, but b) diaries alone are not suitable for tracking HHFW over time or evaluating interventions designed to reduce the amount of HHFW (without substantial further research).
Quested et al. (Fri,) studied this question.