Epidemiology, being primarily an observational rather than experimental science, has often encountered problems with measurement of the study variables, which are not under the control of the investigator. Difficulties arise in part because the variables under study are often subjective and often must be ascertained by subjects' self reports or from records of dubious quality, because biological variability and laboratory error can occur, and because studies are often conducted retrospectively, thus requiring informa tion on events that occurred long ago. Epidemiologists have long recognized that measurement errors have been among the major weaknesses of their studies and have gone to great lengths to try to assess the magnitude of such errors and their likely impact on the conclusions. This problem has spurred much methodologic research, initially on understanding the effects of measurement errors on exposure-response relationships and more recently on developing methods to correct for such errors. The results of the former line of research have often been used qualitatively in the interpretation of epidemiologic findings, but the quantitative predictions have seldom been applied. Thus, the latter line of research appears to be particularly promising in terms of practical applications. Because most of this work is relatively