Selfcontrol design

Within-patient control studies (i.e. crossover and self-controlled studies) or simultaneous within-patient control studies are often used at a preliminary stage in drug development.28 They are also used in dermatology, albeit improperly, at a more advanced stage. In a survey of more than 350 published RCTs of psoriasis (unpublished data), a self-controlled design accounted for one-third of all the studies examined and was relied on at some stage in drug development. The main advantage of a within-patient study over a parallel concurrent study is a statistical one. A within-patient study obtains the same statistical power with far fewer patients, and at the same time reduces variability between the populations being compared. Within-patient studies may be useful when studying conditions that are uncommon or show a high degree of patient-to-patient variability. On the other hand, within-patient studies impose restrictions and artificial conditions which may undermine validity and generalisability of results and may also raise some ethical concerns. The wash-out period of a crossover trial as well as the treatment schemes of a self-controlled design, which entails applying different treatments to various parts of the body, do not seem to be fully justifiable from an ethical point of view. Clearly, the impractical treatment modalities in self-controlled studies or the wash-out period in crossover studies may be difficult for the patient to accept.

Dropouts may have more pronounced effects in a within-patient study than in other study designs because each patient contributes a large proportion of the total information. The situation is compounded in self-controlled studies where dropping out from the study may be caused by observing a difference in treatment effect between the various regions of the body into which the patient has been "divided". In this case, given that dropouts are related to a difference in treatment effect between interventions, the effect of the interventions is likely to be underestimated.

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