Sample

Administrative data on WIC-participating children in Los Angeles County, California, between 2003 and 2016 from the Data Mining Project (https://apps.phfewic.org/Projects/DataMining.aspx) were used and analysed in 2019. These data include weight and height (or length) measures taken on the same children during WIC (re) certification visits, as well as socio-demographic information on children and their families. In addition, children’s residential addresses were geocoded to 2010-census tract boundaries and linked to data from the American Community Survey to obtain neighbourhood-level variables.

The current study was part of a larger study investigating the impact of the 2009 WIC food package change on children’s growth trajectories from 0 to 4 years and obesity at age 4 years⁽²⁾; as such, children’s inclusion criteria were to be continuously enrolled in WIC from birth (defined as within 42 d of birth) until age 4 years (inclusive), with at least one weight and height (or length) measurement for each year, either before or after the implementation of the WIC food package change (1 October 2009). Additional inclusion criteria for the current study were for children to live in a census tract with at least five WIC-participating children to protect the identity of WIC participants. Children who met eligibility were grouped based on the dates they participated in WIC: if their enrollment in WIC ended before 1 October 2009, they belonged to the old food package group, whereas those whose enrollment began after 1 October 2009 belonged to the new food package group. Children whose enrollment in WIC overlapped with the implementation of the food package change were excluded. A total of 155 991 children were eligible for inclusion, with a final analytical sample of 79 502 children attained after sample matching (explained below in the ‘Statistical Analysis’ section; Fig. ​Fig.11).

Flow chart of WIC-participating children included in the study