How does PRISM differ from traditional reliability prediction models?
Traditional reliability prediction models have been based on the operating time of the part, and the units were typically failures per million (or billion) operating hours. The SRC models (and the empirical data contained in the PRISM databases) predict the failure rate in units of failures per million calendar hours. This is necessary because it is the common basis for all failure rate contribution terms used in the model (operating, non-operating, cycling, induced). If an equivalent operating failure rate is desired (in units of failures per million operating hours), the failure rate (Failures/106 calendar hours) can be divided by the duty cycle to yields a failure rate in Failures/106operating hours. Additionally, resulting assembly and system failure rates are modified by process grading factors for the following failure causes; Parts, Design, Manufacturing, System Management, Wear-out, Induced and No Defect Found. These process grades correspond to the degree to which actions have
