Accelerated life-tests for intermittent destructive inspection, with logistic failure-distribution

Summary & Conclusions -Statistically-optimal accelerated life-test plans are suggested for items whose lifetime follows a logistic distribution. Both the scale & location parameters of the lifetime distribution are functions of the stress level. The test plans accommodate intermittent destructive sampling. The number of sampled items which fail to pass the test at the time of each inspection follows a hypergeometric distribution; the number of defective items in the remaining sample which have not yet been tested follows a binomial distribution. Statistically-optimal designs provide test planners with a set of design inputs, such as: 2 stress-levels higher than use stress-level, a set of inspection times, sample allocation, and a censoring time that minimizes the asymptotic variance of the maximum likelihood estimator of a specified quantile of the lifetime distribution. However, such a 2 stress-level optimal plan is not practical because model validation is nearly impossible with so few stress-levels at which to test. In order to overcome such impracticality, compromise plans that require 3 stress-levels are also suggested at a fixed inspection interval -although these plans lose statistical efficiency.