Approaches to Evaluating Welfare Reform: Lessons from Five State Demonstrations. C. TREATMENT OF CROSSOVER CASES

10/01/1996

Even under the best circumstances, a fraction of research cases in an experimental welfare evaluation probably will have their original experimental/control status contaminated (as discussed in Chapter IV). Such contamination can arise for several reasons:

  • Research cases relocate to nonresearch sites and receive different policies.
  • Research cases merge with other cases and receive different policies.
  • Cases split off from research cases and receive different policies.
  • Cases have their experimental/control status altered through administrative error or manipulation.

These cases are commonly called crossover cases, since, in each instance, cases "cross over" from one set of policies to another. Some degree of crossover almost always occurs in a random- assignment evaluation.

Some terms may be helpful in discussing the implications of crossover. Migrant crossover cases are cases that experience a change in experimental/control status as a result of migration to a nonresearch site; merge/split crossover cases are cases that experience a change in experimental/control status as a result of a case merger or split; and administrative crossover cases are cases that experience a change in experimental/control status as a result of administrative error or manipulation. Crossover-type cases are research cases that would be inclined to migrate, merge, split, or otherwise change experimental/control status under at least one of the two sets of policies (experimental and control). Crossover-type cases include actual crossover cases and cases that would have crossed over had they been assigned to the other experimental/control group.