Optimal design of partially accelerated life tests for the exponential distribution under type-I censoring

Optimal designs for two partially accelerated life tests (PALTs) in which items are run at both accelerated and use conditions until a predetermined time are considered. The step PALT allows the test to be changed from use to accelerated condition at a specified time; the constant PALT runs each item at either use or accelerated condition only. For items having constant hazard (failure) rate, maximum-likelihood estimators (MLEs) of the hazard rate at use condition and the acceleration factor, the ratio of the hazard rate at accelerated condition to that at use condition, are obtained. The change time for the step PALT or the sample proportion allocated to accelerated condition for the constant PALT is determined to minimize either the generalize asymptotic variance of MLEs of the acceleration factor and the hazard rate at use condition or the asymptotic variance of MLE of the acceleration factor