The SNAP participation rate is the number of people participating in the Supplemental Nutrition Assistance Program (SNAP) divided by the number of people eligible by income to participate (in Virginia and in most states, this would mean having an income up to 130% of the poverty threshold). The calculation is performed by the U.S. Department of Agriculture's Food and Nutrition Service (FNS) (USDA is the federal funding source for SNAP). The numerator is based on monthly SNAP client data submitted by states to FNS; the denominator comes from population surveys that estimate the number of people living in households with annual income below a specified income-to-poverty ratio. Note: The denominator is a weighted estimate with a degree of uncertainty. The rate is usually expressed as a percentage with a 90% confidence interval (the range within which the true population may lie).
FNS uses a shrinkage estimation method rather than the direct estimation method to improve precision, especially when state sample sizes are small. Using data drawn from various sources (i.e., Current Population Survey, American Community Survey, state SNAP administrative records), the shrinkage estimator averages direct sample estimates of participation rates with predictions from a regression model. To further improve precision, the shrinkage estimator uses three years of data for all states, for both groups (all eligible individuals and the working poor) to derive each state estimate.
There are two data limitations: 1) The Census Bureau provides only estimates of the number of individuals who have an income up to 125% of the poverty threshold, not 130% as the SNAP program requires; 2) Because the CPS/ACS data releases occur approximately 12 months after the end of the reporting period, FNS delays releasing its own rate calculations. For example, the latest SNAP participation rate estimates, made available to states in December 2014, are for CY 2011. The 2012 rates are likely to come out later in 2015.