Selection criteria for psychotherapy outcome studies

The choice of selection criteria for a psychotherapy outcome study depends, of course, on the nature of the research question to be asked. From a public health perspective, samples are usually chosen based upon the presence of a discrete disorder or problem that has significance to society. The selection of the target disorder, however, is only the beginning of the selection process. For studies of DSM Axis I non-psychotic disorders, it is typical that other major psychotic disorders such as schizophrenia and bipolar disorder are excluded from the study. However, there is wide variability across research studies in the extent to which other Axis I and Axis II disorders are included in a study or not.

This aspect of selection criteria relates primarily to the internal versus external validity distinction discussed above. Studies that emphasize internal validity will probably exclude many comorbid diagnoses, while studies that maximize external validity will tend to be more inclusive. As the comorbidities among Axis I diagnoses can be high, the impact on the nature of the patient sample selected can be considerable.

Naturalistic studies that focus on psychotherapy per se, rather than public health concerns, are oriented towards external validity and typically have do not have restrictive selection criteria. For these studies, the question is 'how effective is psychotherapy for the types of patients that end up in psychotherapeutic treatment in the community?' Thus, few if any selection criteria are specified.

One particular selection problem that affects any type of psychotherapy outcome study is whether or not patients currently treated with psychotropic medication are included in the evaluation study. Once again, from the point of view of internal validity—attempting to attribute the treatment outcome to the psychotherapy treatment per se—patients on concurrent medication treatment are usually excluded. In contrast, external validity concerns would lead to the inclusion of patients on medications, since increasing numbers of patients in the community with anxiety and affective disorders are receiving psychotropic medications for their problems. Often, a compromise is struck: patients on medications are eligible for the psychotherapy study as long as they (and their prescribing doctor) agree to maintain a stable dosage of the medication for the duration of the psychotherapy study.

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