CONTEXT Authors have questioned the degree to which medical education research informs practice and advances the science of medical education. OBJECTIVE This study aims to propose a framework for classifying the purposes of education research and to quantify the frequencies of purposes among medical education experiments. METHODS We looked at articles published in 2003 and 2004 in Academic Medicine, Advances in Health Sciences Education, American Journal of Surgery, Journal of General Internal Medicine, Medical Education and Teaching and Learning in Medicine (1459 articles). From the 185 articles describing education experiments, a random sample of 110 was selected. The purpose of each study was classified as description ('What was done?'), justification ('Did it work?') or clarification ('Why or how did it work?'). Educational topics were identified inductively and each study was classified accordingly. RESULTS Of the 105 articles suitable for review, 75 (72%) were justification studies, 17 (16%) were description studies, and 13 (12%) were clarification studies. Experimental studies of assessment methods (5/6, 83%) and interventions aimed at knowledge and attitudes (5/28, 18%) were more likely to be clarification studies than were studies addressing other educational topics (< 8%). CONCLUSIONS Clarification studies are uncommon in experimental studies in medical education. Studies with this purpose (i.e. studies asking: 'How and why does it work?') are needed to deepen our understanding and advance the art and science of medical education. We hope that this framework stimulates education scholars to reflect on the purpose of their inquiry and the research questions they ask, and to strive to ask more clarification questions.