Although the reported incidence of crossover is seldom high for welfare reform waiver evaluations, the evaluations we reviewed had some difficulties in minimizing the risk of crossover. Most research sites were not next to each other, because the desire to obtain a sample representative of the state took precedence over the desire to reduce migration to nonresearch sites. In the four states with experimental evaluation designs, the lack of contiguous research counties may have increased the risk of crossover through migration.
The absence of experimental/control status information in individual records in state administrative systems (as opposed to case records) often made crossover from splits and mergers more likely by failing to identify individuals with previous membership in a research case. In California's evaluation, for example, county-specific automated systems made it difficult for caseworkers to identify crossovers from other counties in the state; the evaluator was able to achieve this identification by relying on state Medicaid records noting receipt of AFDC during the previous 12 months. In Michigan's evaluation, the state was unable to identify the previous research status of individuals reapplying for assistance (although the evaluator later obtained this information by merging case- and individual-level files).