Suppose that a process of production is unstable, i.e. the reliability of units produced in nonintersecting time intervals I1 = (t0, t1], · · · , Iq(tm−1, tm] is diﬀerent: under the same stress conditions the survival functions of units, produced in the intervals Ii and Ij (i = j), are diﬀerent. If the heredity hypothesis (Deﬁnition 2.13) is satisﬁed on E and suﬃciently
large usual and accelerated data are accumulated during a long period of observations, then good estimators of the functions ρ(x(0), x(1)) (or b(x(0), x(1))) can be obtained. The reliability of newly produced units under the usual stress x0 can be estimated from accelerated life data under the stress x(1) > x(0), using the estimators ρˆ(x(0), x(1)) or bˆ(x(0), x(1)) and without using the experiment under the normal stress. We shall follow here Bagdonavicˇius and Nikulin (1997f).