Starting & Limiting Values for Reliability Growth

Modifications of the Duane model for reliability growth which permit formulating test plans prior to test-data availability are presented herein. The modified model yields finite and nonzero MTBFs at the start and end of development testing. The unmodified Duane model is inadequate for formulating the Development Test plan for a Reliability Program because the MTBF is zero at the start of testing and is unbounded for long tests. The parameters of the modified model; limiting MTBF, initial MTBF, and Duane shape parameter (logarithmic growth rate) are estimatable from handbooks and previous experience prior to the start of development tests. An implicit expression for the Duane-model scale-parameter as a function of the limiting MTBF, initial MTBF, and shape parameter is given. The time required and/or the reliability improvement potential of development tests are obtained from the model to plan the reliability program. When the development tests are underway interval estimates for the modified Duane model parameters, needed to monitor the progress of the reliability program, can be computed.