Descriptive studies: what they can and cannot do

Descriptive studies often represent the first scientific toe in the water in new areas of inquiry. A fundamental element of descriptive reporting is a clear, specific, and measurable definition of the disease or condition in question. Like newspapers, good descriptive reporting answers the five basic W questions: who, what, why, when, where ... and a sixth: so what? Case reports, case-series reports, cross-sectional studies, and surveillance studies deal with Individuals, whereas ecological correlational studies examine populations. The case report Is the least-publishable unit in medical literature. Case-series reports aggregate individual cases in one publication. Clustering of unusual cases in a short period often heralds a new epidemic, as happened with AIDS. Cross-sectional (prevalence) studies describe the health of populations. Surveillance can be thought of as watchfulness over a community; feedback to those who need to know is an Integral component of surveillance. Ecological correlational studies look for associations between exposures and outcomes In populations-eg, per capita cigarette sales and,rates of coronary artery disease-rather than in Individuals. Three important uses of descriptive studies include trend analysis, health-care planning, and hypothesis generation. A frequent error In reports of descriptive studies is overstepping the data: studies without a comparison group allow no inferences to be drawn about associations, causal or otherwise. Hypotheses about causation from descriptive studies are often tested in rigorous analytical studies.