For example, software is used in the automobile industry, in aerospace engineering and in medical technology. These scopes are considered to be safety-critical. In the case of failure human life as well as its environment might be in danger. Therefore, software reliability plays a huge role in safety critical applications. They provide information on how long and with the help of which resources a software needs to be tested in order to reach a desired reliability. Currently, there are more than 100 reliability growth models existing. The distinction between the models is based on different assumptions. Most of the models are simply improvements of other models. The littlewood model asserts, errors leading earlier to a breakdown possess a higher hazard rate than errors leading much later to a breakdown. This model should be assumed by means of quality- factor improves the prognosis.