Secondary analysis: theoretical, methodological, and practical considerations.

Secondary analysis, which involves the use of existing data sets to answer new research questions, is an increasingly popular methodological choice among researchers who wish to investigate particular research questions but lack the resources to undertake primary data collections. Much time loss and considerable frustration may result, however, if researchers begin secondary analyses without an awareness of the distinctive methodological and practical challenges involved. This article highlights difficulties that may arise when researchers use data from previous clinical research projects, including theoretical issues and problems involving sampling, measurement, and external and ecological validity. It also offers practical suggestions for undertaking a secondary analysis and criteria for evaluating secondary analyses.