The probability that the total system will perform correctly when it is available is the probability that the hardware/ software will perform correctly, times the probability that the operating environment will not degrade the system operation, and times the probability that the user will perform correctly. By defining the total system this way, human performance is identified as a system component. A system can operate perfectly from an engineering sense in a laboratory or at a demonstration site and then not perform well when the users operate and maintain it at a field location. The Total System Performance will increase significantly by increasing the probability that the operator can perform the task effectively in the appropriate environment. (FAA.gov)