In a multi-version software experiment, twenty programs were built to the same specification of an inertial navigation problem. The programs were then subjected to a three phase testing and debugging process: an acceptance test, a certification test, and an operational test. Less than 20% of the faults discovered during the certification and operational testing were non-unique, i.e. the same or very similar faults would be found in more than one program. However, some of these "common" faults spanned as many as half of the versions. Faults discovered during the certification testing were due to specif cation errors and ambiguities, inadequate programmer background knowledge, insufficient programming experience, incomplete analysis, and insufficient acceptance testing. Faults discovered during the operational testing were of a more subtle nature, and were mostly due to various programmer knowledge defects and incomplete analysis errors. Techniques that may have prevented the observed fault are discussed.
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