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We address problems in modelling the reliability of multiple-version software, and present models intended to improve the understanding of the various ways failure dependence between versions can arise. The previous models, by Eckhardt and Lee (1985) and by Littlewood and Miller (1989), described what behaviour could be expected "on average" from a randomly chosen pair of "independently generated" versions. Instead, we address the problem of predicting the reliability of a specific pair of versions. The concept of variation of difficulty between situations to which software may be subject is central to the previous models cited. We show that it has even more far-reaching implications than previously found. In particular, we consider the practical implications of two phenomena: varying probabilities of failure over input sub-domains or operating regimes; and positive correlation between successive executions of control software. Our analysis provides some practical advice for regulators, and useful insight into non-intuitive aspects of the failure process of diverse software.