Broad Outcome Measures May Underestimate Effectiveness: An Instrument Comparison Study.

Background:   Outcome measures are routinely used in child and adolescent mental health services to demonstrate the effectiveness or otherwise of interventions.

Methods:   We followed up a consecutive sample from a large teaching hospital's Obsessive Compulsive Disorder (OCD) service for young people, comparing improvements using instruments of differing broadness.

Results:   The effect size of improvement decreased as the breadth of the questionnaire increased.

Conclusion:   Specialist clinics are recommended to use two questionnaires: a broad one to facilitate comparability with other clinics, and a narrow one to maximise power to detect significant changes.