, - - - - - - - - - - - - - - - - - - - - - I Epidemiology series I 2002 02 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. Uke 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 publtcation. 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, healtfl.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.