Researcher allegiance and meta-analysis: the case of cognitive therapy for depression.
K. S. Dobson (1989) conducted a meta-analysis of 28 studies of cognitive-behavioral therapy for depression that used the Beck Depression Inventory as outcome measure. He concluded that the outcome of this type of therapy was superior to that of other forms of psychotherapy and to that of pharmacotherapy. The present study reanalyzed the same studies, and a further set of 37 similar ones published from 1987 to 1994, taking into account variations in sample size and researcher allegiance. This study confirmed Dobson's conclusions for his original sample but showed that about half the difference between CT and other treatments was predictable from researcher allegiance. However, comparable analyses of the later set of studies showed no effect of researcher allegiance. Two causes for these phenomena are (a) a historical effect, whereby both effect sizes and allegiance were large in earlier years and declined over time and (b) a treatment effect whereby effect size and allegiance were correlated, but more for some treatments than others. This correlation has also weakened over time.