In other studies they may be contacted at various points for assessment purposes. In some studies no-treatment patients are in weekly phone contact with research staff, who call to check on clinical status and to administer formal assessments. In the latter studies, a considerable placebo effect may occur in no-treatment conditions. These differences between studies are likely to mask a correlation between outcomes that is due to sample differences or other effects. Sample differences among studies can be of considerable magnitude.
Studies vary in the severity of the sample as measured by initial mean depression scores.
They vary in the source of the sample, such as inpatients versus outpatients, symptomatic volunteers versus clinical referrals, and method of selection and exclusion criteria. In some studies subjects are selected on the basis of diagnosis alone. In other studies diagnosis plus a cut-off score on a severity index is required for selection. In the latter studies, the sample will show a regression toward the mean that will not manifest itself in studies with no severity criterion. Sample differences have to do with differences in the medium within which the study is carried out.