Despite the availability of new studies, the indications for surgery have not changed appre- ciably over time.18 Considering that observational studies are prone to bias and confounding, researchers have used regres- sion analysis and calculated propensity scores to adjust for prognostically important baseline differences between surgi- cal and medical patients.14,253 examin- ing the association between valve surgery and outcome in left-sided IE using propensity score analysis, however, have demonstrated conflicting results, likely because of the use of different analytical approaches. Until 2007, none of the published studies adjusted for sur- vivor bias, which occurs because patients who live longer are more likely to undergo surgery than those who die early.A cor- relation between longer survival and surgery may be wrongly interpreted as evidence that surgical treatment improves sur- have documented the effect of survivor bias on the association between surgery and mortality in IE patients. When adjusted for survivor bias, analyses have shown either a statistical loss of benefit of early surgery or findings indicating that the surgical intervention may actually result in harm. Between 2007 and 2013, at least 6 observational stud- that adjusted for selection bias, confounding, and survivor bias were conducted. Three studies that included 2 cohorts of patients with NVE and PVE and 1 cohort with NVE showed an association between early surgery and lower mortality in IE patients in general or in specific subgroups of patients such as those with heart failure or paravalvular com- plications. Only 1 study examined the role of valve surgery After adjustment for differences in clinical char- acteristics and survival bias, early valve replacement was not associated with lower mortality compared with medical therapy in the overall cohort. 