Unless crossover cases leave the state, administrative records on these cases usually will be available even after they migrate to a nonresearch site, split from another case, or merge with another case. Specifically, as long as welfare participation and earnings information are stored in statewide data systems, it will be possible to determine if a research case is participating in welfare or has (UI-covered) earnings. Consequently, researchers will be able to include most crossover cases in the sample used to generate impact estimates.
If crossover cases are included in the analysis sample, the difference in mean outcomes between original experimental cases and original control cases will tend to understate the impact of welfare reform. This dilution results because some cases will have received a mixture of experimental and control group policies. The extent of bias in the impact estimates will depend on the extent of crossover, as well as on the manner in which the impact of welfare reform on crossover-type cases differs from the impact of welfare reform on noncrossover-type cases.
If all crossover-type cases could be identified, then these cases could be excluded from the analysis sample, and impacts could be estimated for noncrossover-type cases only. While these impact estimates would not be representative of impacts of welfare reform on crossover-type cases, they would be unbiased estimates of the impacts of welfare reform on noncrossover-type cases.
In practice, however, crossover-type cases cannot be identified perfectly, since it is uncertain which of the cases in each experimental/control group would cross over if they were subject to the other set of policies. A common practice is to exclude from the analysis sample cases that migrate to nonresearch sites, regardless of their experimental/control status. As long as migration does not depend on experimental/control status, this exclusion will eliminate from the sample both migrant crossover cases and the corresponding group of crossover-type cases. Similarly, to correct for merge/split crossover, all cases that merge or split could be deleted from the sample (although, in practice, it is often difficult to identify all merging or splitting cases within state administrative files).
If crossover behavior does depend on a case's experimental/control status, then excluding crossover cases from the analysis sample will lead to biased impact estimates. The size and direction of the resulting bias will depend on the incidence of crossover and the relationship between experimental/control status and crossover behavior. Biased impact estimates also will result from excluding crossover cases if crossover behavior is correlated with unobserved factors (such as motivation) that affect outcomes. It is not clear whether the bias in impact estimates from excluding crossover cases from the sample exceeds the bias from including these cases in the sample.