Accelerated life models relate the lifetime distribution to the explanatory variable (stress, covariate, regressor). This distribution can be deﬁned by the survival, cumulative distribution, or probability density functions. Nevertheless, the sense of accelerated life models is best seen if they are formulated in terms of the hazard rate function. Suppose at ﬁrst that the explanatory variable x(·) is a deterministic time
function: x(·) = (x1(·), ..., xm(·))T : [0,∞)→ B ∈ Rm.