Introducing Reliability and Availability Requirements into TOC Models
Two important system requirements are reliability and availability. Reliability is the probability of no disabling failures over a certain span of time, while availability is the ratio of the time the system will actually fulfill its operational expectations, to the total time it is expected to fulfill them. Both of these are often included in system specifications.
An explicit reliability requirement may be stated somewhat as follows: The probability of no disabling system failures in 5,000 hours of operations shall be at least 99.9%. An availability requirement may be stated something like this: The system shall be available for operations at least 95% of the times when it is expected to be available.
These requirements are often set with highly imperfect knowledge of their effects on total ownership cost (TOC). The result, if the requirements are rigorously maintained, can be a huge effect on total ownership cost. The effects can be felt in several areas of cost, particularly design labor hours, prototype material cost, production labor hours, production material cost, and support and possibly operational costs after the system is fielded.
This paper explores ways that parametric TOC models are often deficient in addressing failure costs, and how they can be made to better deal with them.
Mr. Stump is a Senior Systems Engineer with Galorath Incorporated. His primary duties are development of logic and algorithms for new cost models or updates of existing ones. He also does cost research. His primary interests are the SEER-EOS, SEER-H, and SEER-MFG models. Mr. Stump has over 55 years of experience in the defense/aerospace industry. He holds the BS degree in Mechanical Engineering from Loyola Marymount University of Los Angeles and the MS degree in Operations Research and Industrial Engineering from the University of Texas at Austin. He is a past president of the Southern California chapters of both ISPA and SCEA.
Ms. Wendy Lee is a Systems Engineer/ Cost Analyst with Galorath Incorporated. Her primary duties are to translate new requirements for cost models into design specification, develops test methodology to validate and evaluate results. She primarily works with SEER-H, SEER-IC, and SEER-EOS models developed by Galorath Incorporated. Ms Lee holds the BS degree in Mechanical Engineering from UCLA and has had over 3 years of experience in developing parametric cost estimating models.