The use of software reliability growth models has become popular. However, the use of most such models is delayed until late in the software product life-cycle (i.e. during testing) as they require failure time data to determine the model parameters. Currently, there is only one model, the basic execution time (BET) model that can be applied earlier in the product life-cycle as its parameters can be related directly to software characteristics. The derivation of the BET model implies uniform execution of program instructions. However, branches and loops in software programs results in non-uniform execution of program instructions. This paper introduces an extension of the BET model, the extended execution time (EET) model that takes into account non-uniform execution of instructions. The non-uniform execution is accounted for by a parameter in the EET model that can be related to measurable characteristics of the software product. This paper further shows the EET model reduces to the BET when instruction execution is uniform and has behavior similar to the Musa-Okumoto logarithmic model in the limit as instruction execution becomes highly non-uniform.
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