Volunteer bias in medical education research: an empirical study of over three decades of longitudinal data

CONTEXT The issue of whether medical education research outcomes can be biased by students' refusal to allow their data to be used in outcomes research should be empirically addressed to assure the validity of research findings. Given that institutions are expected to document the outcomes of their educational programmes, evaluations of clinical performance subsequent to medical school are crucial, but are often incomplete when graduates decline to permit data collection. OBJECTIVES This study aimed to examine the demographic and performance differences between research volunteers and others. METHODS A total of 7415 doctors graduated from Jefferson Medical College between 1970 and 2004; 75% (n = 5575) agreed to participate in medical education research by granting written permission for the collection of data from their postgraduate training directors on their behalf (research volunteers); 20% (n = 1489) refused to grant such permission (non-volunteers), and 5% (n = 351) did not return the permission form (non-respondents). This prospective longitudinal study compared research volunteers, non-volunteers and non-respondents on gender, ethnicity, performance measures prior to, during and after medical school, scores on medical licensing examinations, and board RESULTS Doctors who granted permission (volunteers) generally performed better during and after medical school. In addition, they scored higher on medical licensing examinations and had a higher certification rate. Women and members of ethnic minority groups were less likely to grant permission. CONCLUSIONS The study raises questions about the validity of research findings as a result of volunteer-ism in medical education research. The implications for guidelines regarding the protection of human subjects in medical education research, and for educational outcomes, are discussed.