Symptom Severity Criteria

Historically, minimal levels of symptom severity on rating scales have been incorporated into study inclusion criteria. These criteria are aimed at ensuring that patients have at least a minimum level of symptom severity at baseline to allow for assessing relative effects of a therapy. Issues arise in implementing these inclusion criteria, particularly a possible inflation of rating scale scores to ensure inclusion. This can become an important issue in studies with relatively few patients. To counter possible severity rating inflation, several tactics have been employed, including audio- or videotaping of entry interviews with subsequent quality review; use of self-reporting or an interview rating done outside the study for inclusion; not specifying minimal entry symptom severity criteria to the investigator; and ensuring that there is funding to cover the costs of screening failures.

Whenever entry criteria specify a threshold inclusion, there is the risk of regression to the mean in both the T and C groups. Although regression to the mean (Campbell and Kenny 1999) in an RCT does not bias the comparison of T versus C effects, the use of an entry criterion may focus attention on the very subgroup in which T is least, rather than most, effective, which might lead to a failed RCT. The alternative is to include all those in whom clinicians might use T rather than C (effectiveness criteria) and then use post hoc moderator analyses to determine whether those within a certain range of baseline severity are likely to have the greatest effect sizes.

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