The use of primary care databases: case-control and case-only designs.

Study designs based on the identification of cases are frequently utilized when undertaking epidemiological research. Traditionally these have been mainly based on identification of cases from hospital records. This paper discusses the use of study designs based on the identification of cases focusing on their application to research data derived from primary care. The designs are discussed in the context of using computerized clinical data derived from primary care. The traditional case-control design is considered, with emphasis on the identification of cases and the selection of controls. A common problem when using primary care research databases is that information about potential confounding variables is often limited. Case-only designs, specifically the case-crossover and the within-person case-series, offer alternative designs that aim to overcome problems with confounding. The principles underlying these case-only designs are presented along with examples of their use. The advantages and limitations of the different designs are discussed.