ABSTRACT
Randomized, controlled clinical trials (RCTs) are ideally designed with clear-cut prospective outcomes, on which trial design and statistical power are based. In some cases, patients may continue to be studied after the original RCT is complete. In such instances, extension studies are opportunistic, in that they aim to take advantage of data collected after the prescribed end of the trial for some or all patients originally enrolled. Extension studies thus provide a unique opportunity to collect efficacy and tolerability data and to help generate hypotheses to be tested in subsequent properly designed and implemented prospective RCTs. Extension studies also provide an appealing mechanism for generating data for validating surrogate outcomes capable of predicting long-term change in clinically relevant measures of disability.
