The job performance of men in four different occupational specialties (Vehicle Repairman, Supply Clerk, Armor Crewman, and Cook) was measured by intensive Job Sample Tests of 4 to 5 hours in length and by more conventional Job Knowledge Tests. Curves have been derived that provide estimates of how job performance can be expected to vary as a function of an incombent's aptitude level and amount of job experience. As measured by Job Sample Tests, the performance of men at all levels of aptitude increases with job experience out to approximately five years on the job. During this time, separation between different aptitude groups is generally maintained. Beyond five years, performance of the different groups tends to converge. Scatter plots of these data are used to identify an empirically based, operational definition of job proficiency. (Author)