TY - JOUR T1 - Commentary: What is a propensity score? JF - British Journal of General Practice JO - Br J Gen Pract SP - 687 LP - 687 DO - 10.3399/bjgp08X342471 VL - 58 IS - 555 AU - Jennifer Nicholas AU - Martin C Gulliford Y1 - 2008/10/01 UR - http://bjgp.org/content/58/555/687.abstract N2 - The paper by van Marwijk and colleagues1 illustrates the application of propensity scores to the analysis of a cluster randomised trial. This commentary outlines the role of propensity scores in the analysis of non-randomised studies and randomised trials.Propensity scores in non-randomised studies. Consider the example of a population-based register of angina patients. Suppose that a researcher wishes to compare the long-term survival of patients who received coronary artery bypass surgery (CABG) with those who did not receive surgery. Patients selected for CABG can be expected to differ from those that did not receive surgery in terms of important prognostic characteristics including the severity of coronary artery disease or the presence of concurrent conditions, such as diabetes. A simple comparison of the survival of patients who either did or did not receive CABG will be biased by these confounding variables. This ‘confounding by indication’ is almost invariably present in non-randomised studies of healthcare interventions … ER -