The effect of reparametrization on accelerated lifetime tests

Efficient reliability industrial experiments consist of submitting items to accelerated life tests. It is of interest to obtain measures of the realiability of the devices under the usual working conditions, represented here by the mean lifetime. A practical problem refers to the accuracy of interval estimation of the parameter of interest when the sample size is small or moderate. In this paper, we describe the effect of several reparametrizations on the accuracy of the interval estimation. We propose a reparametrization that leads to accuracy while allowing orthogonality between the parameters. The idea is to consider a logarithmic reparametrization on orthogonal parameters in order to have independent maximum likelihood estimates with good asymptotic normal approximation. The study is illustrated by a data set on an accelerated life test at pressurized containers of Kevlan/Epoxy 49.