head-to-head comparison, within study comparison
Treatment comparisons within a study are direct comparisons, whereas treatment comparisons between studies are indirect comparisons.
For example, if two drugs are each compared to a placebo in different studies, but not compared to each other in any studies, there are direct comparisons of each drug to placebo, and an indirect comparison of the two drugs is possible, although there is not a direct comparison of the two drugs.
In a direct comparison of diagnostic tests using a paired design, an index test and comparison test are both evaluated in the same study in relation to the reference standard. In an indirect comparison, the index test and comparison test would each be compared to the reference standard in separate studies.
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