Conceptually and algebraically defined measures of relative regimen and program effectiveness also are presented. The measures evaluate preventive services that range from individual tests through organized, population-wide prevention programs. Examples illustrate how effective screening regimens may not result in effective screening programs and how measures can vary across subgroups and settings. Both regimen and program relative effectiveness measures assess benefits of prevention services in real-world settings, but each addresses different scientific and policy questions. As the body of CER grows, a common lexicon for various measures of relative effectiveness becomes increasingly important to facilitate communication and shared understanding among researchers, healthcare providers, patients, and policymakers.