Estimating psychological treatment effects from a randomised controlled trial with both non-compliance and loss to follow-up

The Outcomes of Depression International Network (ODIN) trial evaluated the effect of two psychological interventions for the treatment of depression in primary care. Only about half of the patients in the treatment arm complied with the offer of treatment, prompting the question:'what was the effect of treatment in those patients who actually received it?' To illustrate the estimation of the effect of receipt of treatment in a randomised controlled trial subject to non-compliance and loss to follow-up. We estimated the complier average causal effect (CACE) of treatment. In the ODIN trial the effect of receipt of psychological intervention (an average of about 4 points on the Beck Depression Inventory) is about twice that of offering it. The statistical analysis of the results of a clinical trial subject to non-compliance to allocated treatment is now reasonably straightforward through estimation of a CACE and investigators should be encouraged to present the results of analyses of this type as a routine component of a trial report.