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).