In avionics and other critical systems domains, adequacy of test suites is currently measured using the MC/DC metric on source code (or on a model in model-based development). We believe that the rigor of the MC/DC metric is highly sensitive to the structure of the implementation and can therefore be misleading as a test adequacy criterion. We investigate this hypothesis by empirically studying the effect of program structure on MC/DC coverage.

To perform this investigation, we use six realistic systems from the civil avionics domain and two toy examples. For each of these systems, we use two versions of their implementation-with and without expression folding (i.e., inlining). To assess the sensitivity of MC/DC to program structure, we first generate test suites that satisfy MC/DC over a non-inlined implementation. We then run the generated test suites over the inlined implementation and measure MC/DC achieved. For our realistic examples, the test suites yield an average reduction of 29.5% in MC/DC achieved over the inlined implementations at 5% statistical significance level.