Planning accelerated life tests with random effects of test chambers

In accelerated life tests (ALTs), test units are often tested in multiple test chambers along with different stress conditions. The nonhomogeneity of test chambers precludes the complete randomized experiment and may affect the life-stress relationship of the test product. The chamber-to-chamber variation should be taken into account for ALT planning so as to obtain more accurate test results. In this paper, planning ALTs under a nested experimental design structure with random test chamber effects is studied. First, by a 2-phase approach, we illustrate to what extent different test chamber assignments to stress conditions may impact the estimation of unknown parameters. Then, D-optimal test plans with 2 test chambers are considered. To construct the optimal design, we establish the generalized linear mixed model for failure-time data and apply a quasi-likelihood method, where test chamber assignments, as well as other decision variables that are required for planning ALTs, are simultaneously determined.