Investigator allegiance and the evaluation of psychotherapy outcome research

Considerable evidence has demonstrated that the beliefs of researchers can inadvertently influence research findings. The possibility of this type of bias is of special concern in studies evaluating the outcome of psychotherapy, where investigators frequently have marked allegiances to particular therapies and these allegiances have been found to correlate substantially with the pattern of results. In this article we discuss the evidence concerning investigator allegiance in psychotherapy research, emphasize the need to distinguish between this factor as a potential confound and a proved causal effect, and outline strategies that have been suggested for researchers to minimize the potential for bias both when designing future research and drawing conclusions from existing evidence.