A major problem for Human Factors is implementing effective human performance consideration into system design; presenting the data in engineering, not psychological, terms. The most usual basic engineering requirement is for information which can be handled in mathematical terms. Mathematical description of human performance is difficult because of the non-linear and time-varying characteristics of man. The human factors mathematical data needed for system Conceptual Phase engineering solutions are generalized models, rules of thumb, or sovereign factors. During the Design Phase human task performance, and especially task complexity, should be quantified. During System Acquisition Phase, operation failure and error analyses must include valid human error probability determination. Methods of meeting these requirements are suggested.