Outcome swapping

When a clinical trial has a protocol published in advance, it will typically include pre-specified primary outcomes: these are the main measures used to determine if an intervention or drug is effective. For example, change in average pain level measured with a questionnaire, biological test results or physical measurements of ability.

Outcome swapping is the process of changing the primary outcomes during the trial or prior to publication. For example, a secondary outcome may be changed with a primary outcome, which may give then change the results reported in the trial publication. In the PACE trial, outcome swapping led to results showing that both CBT and GET were moderately effective, but when a secondary analysis used the original pre-specified outcomes this showed the interventions were less effective than the previously published results.