The welfare reform waiver evaluations we studied had different approaches to including crossover cases in the analysis sample.
Wisconsin's WNW evaluation employed a comparison group design; the only crossover that could arise would involve migration out of a demonstration county. The demonstration counties chosen for the evaluation were small, so crossover to nonresearch counties (that is, counties without provisions such as time limits) was a major concern to the evaluator. The risk of crossover to the comparison counties was reduced somewhat by selecting noncontiguous counties for the evaluation, but crossover to other counties with pre-reform policies remained a concern. Efforts were being made to keep track of cases that had left a particular demonstration county for another location in the state.
For the California evaluations, the evaluator is attempting to track crossover cases throughout the state. AFDC and Medicaid participation information is available for the entire state, but AFDC benefit information is available for cases in the research counties only, which limits the ability to include crossover cases in certain impact analyses.
The Colorado evaluation excluded migrant crossover cases from the analysis, but only if they continued to receive welfare in nonresearch counties. If rates of migrant crossover or subsequent welfare receipt differed for experimental and control cases, then this practice would lead to biases in impact estimates. Merge/split crossover cases were always excluded from the Colorado analysis sample. To the extent that merge/split crossover rates differed for experimental and control cases, this would also lead to biases in impact estimates (although the incidence of merge/split crossover is typically small).
Michigan's evaluation deleted crossover cases and any other cases that left the research sites from the research sample. After processing data corresponding to the three years following the implementation of welfare reform, about one-fifth of the total research sample had been deleted for these reasons (in approximately equal percentages for experimental and control cases). To make the resulting sample more representative of the original research sample, a system of weights was developed that controlled for research site, recipient/applicant status, year of application, and number of adults in the case at baseline. All impact estimates in the fourth annual report (including third-year impacts for recipient cases) were estimated using these weights.
In Minnesota's evaluation, crossover cases were included in the analysis sample. Following standard MDRC procedures, impact estimates were generated using original experimental/control status, without employing the Bloom procedure or other corrections for crossover. The MDRC approach provides a precise lower-bound estimate of the impact of welfare reform on cases in the research sample. No analyses were reported showing the sensitivity of impact estimates to use of the Bloom correction or exclusion of actual crossover cases from the research sample.