How to model reliability-growth when times of design modifications are known

Engineers often make changes during system development in order to correct design weaknesses. If done well this results in reliability growth as development continues. Since no reliability-growth model is adequate for all time (any growth model is valid only for some finite amount of time) it is prudent to use a different reliability model between each two design modifications. This paper assumes that the reliability growth model between each two design modifications is either a Poisson process or power-law process. Maximum likelihood estimates of the parameters are obtained for two cases: (1) no constraints on parameters; and (2) some constraints on parameters.