In addition to these more standard analyses, one can obtain, through simulation, estimates of answers to any additional well-framed questions concerning states, arcs, paths, and so on. Also, one can specify transition probabilities by developing a system of constraints, that is, linear equalities and inequalities involving the transition probabilities, and then optimizing an objective function using linear programming.

Discussion of Dalal and Poore Papers

In the discussion of these papers, Margaret Myers asked software experts to examine four questions: (1) How does one estimate the reliability of a software system that is structured as a series of component software systems? (2) How does one address the integration of commercial off-the-shelf software systems, and how does one estimate the reliability of the resulting system? (3) How does one estimate reliability for a system that is being acquired in a spiral environment? (4) What sort of regression testing is useful in evolutionary acquisition?

Jack Ferguson pointed out that software methods are extremely important to DoD. DoD spends approximately $38 billion a year on research, development, testing, and evaluation of new defense systems, and it is estimated that approximately 40 percent of that cost is for software. Any method that can help DoD to make even a small improvement in software development can represent a large amount of savings. However, it should be kept in mind that the software is not always the problem. DoD is using software to do more and more, mainly to provide the flexibility required to meet new environments of use. Therefore, the problem is often fundamentally a domain-analysis problem.

Ferguson added that given the widespread use of commercial off-the-shelf systems, it is important to consider the use of black-box testing, as discussed in Poore’s (and Dalal’s) presentation. The workshop devoted to software reliability should devote a good deal of time to these methods.

Ferguson remarked that the traditional way of determining software reliability required a great deal of inspection, which is no longer workable. It was stressed that models such as those used by Dalal and Poore need to be developed as early as possible in system development, since, as with hardware systems, it is generally much less costly to fix a problem discovered early in the design phase.

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