Burn-in for Heterogeneous Populations

Abstract

In the previous chapters, we discussed the burn-in procedures for homogeneous populations. When the failure rate of a population is decreasing or bathtub-shaped (BT), burn-in can be usually justified. Note that, as mentioned and illustrated earlier, the heterogeneity of populations is often a reason for the decrease in the resulting failure rate, at least, in some time intervals. In this chapter, the optimal burn-in procedures are investigated without assuming that the population failure rate is BT. We consider the mixed population composed of two ordered subpopulations—the subpopulation of strong items (items with ‘normal’ lifetimes) and that of weak items (items with shorter lifetimes). In practice, weak items may be produced along with strong items due to, for example, defective resources and components, human errors, unstable production environment, etc. In the later part of this section, we will also consider the continuous mixtures model.