A danger in matched-control studies.

IN READING the medical research literature, one often sees references to or use of matched controls. This procedure may be included, for instance, in comparative drug trials or studies of the putative etiologic effect of certain environmental factors. Actually, there are a variety of research settings that may lend themselves to this approach. What do we mean by matching? Briefly, it ordinarily consists of the selection, for each test subject in a study (eg, one with a certain disease or receiving a certain therapy), of a comparison subject who is as similar to the test subject as possible except for the presence of the disease or treatment under investigation. One of the ways in which matching differs from stratification or blocking is that the comparison subjects in matching are selected individually to match individual test subjects. For example, in a case-control study of the possible effect of fiber consumption in