One of the main concerns in safety-critical software is to ensure sufficient reliability because proof of the absence of systematic failures has proved to be an unrealistic goal. fault-tolerance (FT) is one method for improving reliability claims. It is reasonable to assume that some software FT techniques offer more protection than others, but the relative effectiveness of different software FT schemes remains unclear. We present the principles of a method to assess the effectiveness of FT using mutation analysis. The aim of this approach is to observe the power of FT directly and use this empirical process to evolve more powerful forms of FT. We also investigate an approach to FT that integrates data diversity (DD) assertions and TA. This work is part of a longer term goal to use FT in quantitative safety arguments for safety critical systems.