An analysis of the effect of inaccuracy in the descriptive inputs with which the cost analyst must work. The degree to which the output is affected is demonstrated by an examination of three of the standard inputs in cost analyses of fixed-wing aircraft: maximum speed, gross takeoff weight, and maximum thrust. Estimates of each, based on information from the Air Force historical files, show that they are marked by substantial error and that two reveal substantial underestimates or bias. When used in a sample cost analysis, the inaccurate inputs change the results by as much as 36 percent, or $2.6 billion, below the actual cost. Major advances in cost analysis accuracy will require careful consideration of the inputs as well as the models and outputs. (Presented to the 3rd Annual DOD Cost Research Symposium, March 13-15, 1968.) 20 pp.