Power to detect homework effects in psychotherapy outcome research.

Studies of homework effects in psychotherapy outcome have produced inconsistent results. Although these findings may reflect the comparability of psychotherapy with and without homework assignments, many of these studies may not have been sensitive enough to detect the effects sizes (ESs) likely to be found when examining homework effects. The present study evaluated the power of homework research and showed that, on average, current power levels are relatively weak in controlled studies ranging from 0.58 for large ESs to 0.09 for small ESs. Thus, inconsistent findings between studies may very well be due to low statistical power.