From the AOBS Chairman - Evaluating the Exam, the Examinee and the Examiner: There is No Curve 

All written, oral, and clinical osteopathic board certification examinations are criterion referenced.   
The AOBS has adopted an extensive evaluation process whereby all individual written and oral test items (and even examiners) are reviewed to maintain high quality standards. Through detailed computer analysis, the AOBS can determine how well an item performs and whether or not it differentiates those with the requisite special knowledge associated with their area of practice by choosing the correct answer and those who cannot. It is possible to identify items that are too easy or too hard, items that unintentionally may have two plausible answers, or items that may have been incorrectly keyed. When the entire exam is evaluated, the AOBS uses computer analysis and psychometrician expertise to determine how reliable the test is and how well the test items differentiate the high performers (pass) and low performers (fail). Regardless of the statistical analysis, all documents for candidates who perform below but within a standard error of the cut score or pass point are reviewed by hand by all AOBS members in that discipline for mechanical and unintended errors.   

Timothy M. Burandt, DO, FACOS 
Chairman, American Osteopathic Board of Surgery