Seattle's minimum wage ordinance did not affect supermarket food prices by food processing category

Objective: To examine the impacts of Seattle's minimum wage ordinance on food prices by food processing category. Design: Supermarket food prices were collected for 106 items using a University of Washington Center for Public Health Nutrition market basket at affected and unaffected supermarket chain stores at three times: March 2015 (1-month pre-policy enactment), May 2015 (1-month post-policy enactment) and May 2016 (1-year post-policy enactment). Food items were categorized into four food processing groups, from minimally to ultra-processed. Data were analysed across time using a multilevel, linear difference-in-differences model at the store and price level stratified by level of food processing. Setting: Six large supermarket chain stores located in Seattle (''intervention') affected by the policy and six same-chain but unaffected stores in King County (''control'), Washington, USA.SubjectsOne hundred and six food and beverage items. Results: The largest change in average price by food item was +$US 0.53 for ''processed foods' in King County between 1-month post-policy and 1-year post-policy enactment (P < 0.01). The smallest change was $US 0.00 for ''unprocessed or minimally processed foods' in Seattle between 1-month post-policy and 1-year post-policy enactment (P = 0.94). No significant changes in averaged chain prices were observed across food processing level strata in Seattle v. King County stores at 1-month or 1-year post-policy enactment. Conclusions: Supermarket food prices do not appear to be differentially impacted by Seattle's minimum wage ordinance by level of the food's processing. These results suggest that the early implementation of a city-level minimum wage policy does not alter supermarket food prices by level of food processing.