Frequently two or more treatments for the same disorder, as practised in different settings, are contrasted. In principle, such a design could answer the question "what kind of patient benefits most from particular treatment protocols". In reality differences in case-mix and the failure to control specific components of treatment usually place drastic limitations on the implications which may be drawn from such studies. Given a sufficiently large data-set, it may be possible to derive conclusions about the relative value of treatments even in the absence of random assignment. However, studies on such a large scale are rarely possible. 506) 4'20&&;443&&32D.2E&&F&&19.3;&&2;.1.40&&GHIJ&&F& OQ
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