Applied MethodsDistinguishing Selection Bias and Confounding Bias in Comparative Effectiveness ResearchHaneuse, Sebastien PhDAuthor Information Department of Biostatistics, Harvard School of Public Health, Boston, MA Supported, in part, by grant R01 MH083671 from the National Institutes of Mental Health (NIHM; PI David Arterburn). The author declares no conflict of interest. Reprints: Sebastien Haneuse, PhD, Department of Biostatistics, Harvard School of Public Health, 655 Huntington Ave., SPH2, Floor 4, Boston, MA 02115. E-mail: email@example.com. Medical Care: April 2016 - Volume 54 - Issue 4 - p e23-e29 doi: 10.1097/MLR.0000000000000011 Buy Metrics Abstract Comparative effectiveness research (CER) aims to provide patients and physicians with evidence-based guidance on treatment decisions. As researchers conduct CER they face myriad challenges. Although inadequate control of confounding is the most-often cited source of potential bias, selection bias that arises when patients are differentially excluded from analyses is a distinct phenomenon with distinct consequences: confounding bias compromises internal validity, whereas selection bias compromises external validity. Despite this distinction, however, the label “treatment-selection bias” is being used in the CER literature to denote the phenomenon of confounding bias. Motivated by an ongoing study of treatment choice for depression on weight change over time, this paper formally distinguishes selection and confounding bias in CER. By formally distinguishing selection and confounding bias, this paper clarifies important scientific, design, and analysis issues relevant to ensuring validity. First is that the 2 types of biases may arise simultaneously in any given study; even if confounding bias is completely controlled, a study may nevertheless suffer from selection bias so that the results are not generalizable to the patient population of interest. Second is that the statistical methods used to mitigate the 2 biases are themselves distinct; methods developed to control one type of bias should not be expected to address the other. Finally, the control of selection and confounding bias will often require distinct covariate information. Consequently, as researchers plan future studies of comparative effectiveness, care must be taken to ensure that all data elements relevant to both confounding and selection bias are collected. Copyright © 2016 Wolters Kluwer Health, Inc. All rights reserved.