- Sampling WIC Service Sites. We plan a two-stage sampling approach that uses the WIC 2010 Participant Characteristics data (WIC PC 2010) to develop the WIC site sampling frame and a stratified sample design to select the sample of sites. In the first-stage we will use a group of characteristics to stratify the WIC sites into 40 strata. Because of uncertainties about the eligibility of the first-stage sampling units, these units will be selected in two phases. In the first phase a total of 160 sampling units in 42 State Agencies will be selected—4 from each of the 40 strata. After the phase 1 selection, we will list the service sites associated with each first-stage sampling unit selected and determine the eligibility of each unit. To be eligible for the study, a site must have an average minimum daily flow of 1.5 new WIC ITFPS-eligible enrollees per day and must be expected to remain in operation and enrolling new WIC participants during the WIC ITFPS recruitment period. In the second phase we will subsample eligible first-stage sampling units to arrive at the final sample of 80 first-stage sampling units (2 from each of the 40 strata). In first-stage sampling units that are local agencies with more than one eligible service site, a second stage of sampling will be conducted to select one service site. The final sample will consist of 80 eligible service sites. Once the second-stage sampling is complete, recruitment efforts will begin in earnest. Although due diligence will be used to recruit service sites, we anticipate that some sites may be unable or unwilling to cooperate. Such service sites will be replaced by members of a matched sample. This replacement of service sites by matched substitutes is similar to imputation.