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suspicion of an association and, in fact, has been instrumental in identifying certain adverse effects (e.g., diethylstilbestrol and vaginal adenocarcinoma). In fact, much of the available data on adverse outcomes and pharmaceutical compounds comes from case series designs. Cross-sectional studies measure exposures and outcomes at the same point in time. Correlational or ecological studies attempt to correlate an exposure with an outcome. Individual case studies or case series also are used. Most descriptive studies compare disease or health-related endpoints in relationship to a specific exposure or risk factor. Because comparison groups vary with regard to other factors associated with the exposure, further assessment of associations is needed, and causality cannot be determined.

Analytical studies include cohort (prospective and retrospective) and case-control (retrospective) types. Several hybrid designs exist as well, such as those that use retrospective cohorts. Control studies might be matched or unmatched in the design phase; matched-cohort studies are relatively rare, despite offering improved efficiency over other designs (K.J. Rothman and Greenland 1998). The major distinction between cohort and case-control designs is that cohort studies begin with the exposure and follow individuals to ascertain incident or new cases of disease. In this regard, the investigator has confidence in the temporal ordering between exposure and outcome. Case-control studies, on the other hand, start with disease status and retrospectively ascertain exposure, and may be subject to biases associated with the correction of data.

Experimental designs include randomized clinical (or community) trials and are considered the most scientifically desirable design available to epidemiologists. These designs ensure the temporal ordering between an exposure and outcome and minimize confounding via the randomization process by maximizing the internal validity of findings; external validity may be limited. Such designs have limited applicability to environmental and occupational epidemiology, given that exposures typically cannot be randomly assigned. Few “natural” experiments occur in which a particular subgroup of the population is exposed while others are not, and exposure is not randomized in such instances.

Scientifically sound epidemiological studies adhere to the essential elements of the epidemiological method:

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