This paper analyzes data related to working hours that have been recorded over seven years, during the development of a real-life software system. The software under consideration is composed of two diverse (dissimilar) units, called variants. The second variant is used for self-checking. The results of the two variants are compared: in case of agreement, the outputs of the principal variant are transmitted and in case of discrepancy an error is reported. For each development phase, we evaluate the cost overhead induced by the development of the second variant with respect to the cost of the principal variant, considered as a reference. The results show that the cost overhead varies from 25 % to 134 % according to the development phase.