For products whose failures are defined in terms of performance characteristics exceeding their critical values

reliability assessment can be based on degradation measurements by using degradation models.Since the time-to-failure depends on the level of critical value

more life data can be obtained by tightening the critical value.This paper presents a method for the estimation of life distribution for long life products by using life data from degradation measurements.The relationship between life and critical value and stress is modeled and used to estimate the life distribution at a usual critical value design stress.The model parameters are estimated by using maximum likelihood and least square method.At last

an example is used to illustrate the validity of the method.