Current literature on the safety and efficacy of freestanding birth centers suggests that these centers are safe and have reduced costs for delivery of low-risk women when compared with hospitals. Despite these findings, birth centers continue to arouse controversy and remain limited in number. Potential inequality of birth center and hospital subjects as to perinatal risk is cited as the major methodologic flaw in the current research on birth centers. Defining an appropriate comparison group is arguably the most important methodologic issue encountered in these investigations. Defining women as “low risk” according to standard perinatal risk tools is not an adequate measure of comparability, as these criteria are generally not equivalent to those defining birth center eligibility. The key is to identify groups for comparison that, at baseline, would be expected to have similar outcomes. To address this concern, a tool based on the American College of Nurse-Midwives'Nurse-Midwifery Clinical Data Set was developed to identify valid comparison subjects for birth center research. This tool focuses on birth center eligibility as opposed to traditionally defined risk. This article reviews the issues of population comparability in birth center research and presents the results of a validation study using this newly developed tool.