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Scanless diagnosis of a lumbar disc protrusion

been assembled and defined on the basis of exposure or non-exposure to a putative risk factor for disease. In such a study, the cohort has been observed for a period of time, during which the occurrence of disease (or outcomes) has been noted.! The study by Strasser et al. is not an example of a cohort study. The study population comprised hepatitis C-virus-positive patients in Victoria and the major outcome of interest to the investigators was the presence of abnormalities on percutaneous liver biopsy. Risk factors under consideration in this study included age, sex, ethnicity, alcohol intake, injecting drug use and blood transfusion. This was quite clearly a cross-sectional study (or prevalence survey), in which information on exposure and outcome is collected simultaneously (or as near as practicable). The point is not merely academic, as the inferences we draw from epidemiological studies are wholly dependent upon the techniques used in the collection and analysis of data. The descriptor "cohort" conveys a precise meaning within the discipline of epidemiology. At the risk of oversimplification, the findings from a cohort study provide a higher level of evidence than the findings from a cross-sectional study for assessing the strength of any relationship between exposure and outcome. Because of this, a cohort study is accorded great significance by epidemiologists, and presumably the wider medical community as well. A correct description of the study design is therefore essential not only for interpretation of data, but also as a matter of principle, particularly when it features prominently in titles and abstracts of research papers. To do otherwise is misleading.