Details of Sampling and Eligibility Considerations

Appendix VV

Details of Sampling and Eligibility Considerations

As noted in Section B.2.1, among mothers who enroll at each service site during the site’s recruiting window, two samples will be selected, a core longitudinal and supplemental cross-sectional sample. The core sample will be an equi-probability of all new enrollees. The supplemental sample will focus on rarer population subdomains such as black mothers and infants who have no prenatal WIC exposure. The supplemental sample will not be designed to be analyzed by itself but only in conjunction with the core sample. The two samples will start out as equal in size with an average 49 new enrollees each per service site; recruitment will alternate between the core sample and the supplemental sample. During recruitment we will screen the supplemental sample on race, ethnicity, trimester at enrollment, pre-pregnancy BMI, household composition, and income, dropping women and infants not required to achieve the subgroup targets. The supplemental subsampling rates are based on the sample sizes needed to support the precision requirements (power projections) discussed in Section B.2.3, and were determined taking into account estimated population distributions. The supplemental sample will end up being considerably smaller after screening and subsampling. A 10 percent rotating subsample of the core sample will be drawn for purposes of administering second-24HR recalls within a week of each interview at 13, 15, 18, and 24 months.

In addition to past WIC enrollment for the same pregnancy/infant, another reason for immediate ineligibility is that the infant is older than 10 weeks at the time of enrollment. Table B1.2 shows our projected sample sizes and response rates at various recruitment and interviewing stages over time. The sources of data and approach used to obtain these estimates (as well as other population estimates used to develop the ITFPS-2 sample design) are discussed in Section B.2.3.

When infants are enrolled by someone other than their biological mother, we will make the person filling out the enrollment papers the primary respondent for the survey if they report that they will be the primary caregiver for the infant. The situation is more complicated if someone other than mother fills out the enrollment papers but says that the mother will still be the primary caregiver because we need informed consent from the targeted primary respondent. We cannot accept proxy informed consent. For those WIC mothers who have twin, triplets, and so on, we will sample a single infant (randomly from the multiples) at the first postnatal interview.

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