The test process of safety-critical systems underlies the challenges of growing complex systems and still limited means to satisfy the functionality of a system under test against its requirements. The whole test process has to fulfill the criteria of reliability, objectivity and traceability, because its results act as verification for the correct functionality of the safety-critical system to the public authority. The classification tree method is a common method to support the test process in a model based way. Unfortunately, the actual implemented variants of this method break either the reliability or the traceability. This article presents an extension to the classification tree method to satisfy both, the reliability in the classification tree results and the traceability in the estimation of the adequate test cases. The application of the implemented extension is shown as an example.