As part of a longitudinal study of the growth of general medical knowledge among osteopathic medical students, a simple, convenient, and accurate vertical equating method was developed for constructing a scale for medical achievement. It was believed that Parts 1, 2, and 3 of the National Board of Osteopathic Medical Examiners' (NBOME) examination together represented the basic concepts and principles of the entire medical sciences, so all 3 (comprising 9 examinations in all) were combined in a linking chain into a large examination representing 2,814 items and 5,168 persons, although there was substantial missing data. The Rasch model measurement program BIGSTEPS was used to calibrate this huge examination. Despite the large amount of missing data, the program converged smoothly. Distributions of person ability were not affected by the equating. Results suggest that Rasch measurement one-step equating is a valid, efficient, and accurate way to construct a measure for longitudinal medical achievement studies. The one-step technique accomplished by BIGSTEPS provided good person ability estimates on the whole examination, and consistent difficulty estimates for items. Ten figures and two tables present details of the analyses. (SLD)