Likelihood Function of a Debugging Model for Computer Software Reliability

A simple model for software reliability growth, originally suggested by Jelinski & Moranda, has been widely used but suffers from difficulties associated with parameter estimation. We show that a major reason for obtaining nonsensical results from the model is its application to data sets which exhibit decreasing reliability. We present a simple, necessary and sufficient condition for the maximum likelihood estimates to be finite and suggest that this condition be tested prior to using the model.