[Clin Psychol Sci Prac 18: 119–125, 2011]
“Allegiance bias” has been hypothesized to compromise the findings of randomized controlled trials (RCTs). In contrast, our multisite RCT involving the collaboration of investigators with different allegiances regarding interpersonal psychotherapy (IPT), guided self-help cognitive behavior therapy (CBTgsh), and behavioral weight loss therapy (BWL) for binge eating disorder showed no evidence of any differential site × treatment effects. The findings indicate that “allegiance bias” does not necessarily occur in well-controlled RCTs with appropriate therapist training. We also examined the role of individual therapist differences that have been alleged to be more important than treatment effects. No individual therapist effects emerged on any measure in either IPT or CBTgsh, both of which were significantly more effective than BWL at two-year follow-up.