Longitudinal studies 4: Matching strategies to evaluate risk.

Matching is a strategy that can be used to control for confounding at the design stage of observational studies that examine exposure-outcome relationships. In case-control studies, matching can be used to generate subsamples of case and control units that are similar with respect to one or more confounders. In cohort studies, matching can balance confounder(s) so that they are the same in exposed and unexposed groups. Matching methods have been extended to include multivariable approaches, the most common being propensity score matching in observation studies of interventions. This chapter describes the major principles of matching applied to case-control, cohort, and propensity score studies. Matched study designs provide several advantages for controlling confounding in observational studies; however, they remain vulnerable to residual confounding and can even introduce bias when implemented incorrectly.