groups occurs, the assumption is that the groups will be equivalent. However, with a relatively small sample size, which is the case for most studies of intervention effectiveness, it is essential for the researcher to confirm that participants in different groups are equivalent on major variables that might affect outcome. If participants are vaguely described, then there is limited information about the equivalence of comparison groups.

The recruitment, selection, and attrition of participants are also important issues. Standards and expectations for reporting how potential research participants were identified and persuaded to participate, how they were selected from the pool of potential participants, and how many participants completed the study have been very different within different disciplines (e.g., experimental psychology and epidemiology) and different perspectives (e.g., developmental and behavioral). With increasing attempts to integrate perspectives (see Filipek et al., 2000) to produce practical guidelines or meta-analyses, this information becomes crucial. For example, it is much more difficult to interpret results of a meta-analysis of success rates when a potentially large number of participants proposed for the research may have not been selected because they were deemed likely to be poor responders to an intervention, and another significant proportion of participants may not have completed their course of treatment. If samples are to be combined, and if interpretations are going to span fields, then there will be a need for more information about these processes.

Researchers are often interested in the interactions between child or family characteristics and treatment, sometimes referred to as aptitude-by-treatment interactions. Such analyses allow researchers to determine if the intervention was more effective for participants with certain characteristics. For example, one type of comprehensive treatment program might produce more positive outcomes for children who communicate verbally than for children who are nonverbal. The analysis requires that a reliable measure of the child characteristic or “aptitude” variable be collected. Vague participant descriptions could preclude the possibility of such analyses.

General, nonstandard participant descriptions also affect the external validity of studies (i.e., the degree to which the findings of a study can be generalized to other individuals not in the study [Campbell and Stanley, 1963]). To interpret for whom an individual intervention procedure or comprehensive intervention program might be effective, one has to have a clear understanding of who participated in the study. Both single-subject and group studies build their evidence for external validity on study replications. To compare the findings of different studies, researchers must be able to determine that children with similar characteristics participated in the study.

In many studies of children with autistic spectrum disorders, descrip-



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