system failure. Of course, models that make no use of direct understanding of specific failure modes can still be useful in certain broad contexts, but their validity is, generally speaking, more questionable.

To make progress in the development and implementation of physics-of-failure models will require the interaction of statisticians and other scientists. Some systems or components will not benefit from this type of approach since the physics underlying the phenomenon, for one reason or another, is not mature enough. For many different types of defense systems, however, even partial knowledge of the underlying mechanism for various failure modes can be extremely helpful to assist in the statistical estimation of reliability. Some areas addressed by the workshop in which these points were made were (1) fatigue modeling, to help apply the proper acceleration function; (2) early assessments of system reliability for PREDICT; (3) help in categorizing failure modes into types A and B in the Integrated Reliability Growth Strategy (IRGS); (4) improved understanding of whether combining information from developmental and operational test is reasonable; and (5) Meeker’s research linking developmental, operational, and field use by employing various acceleration models.

A related issue is separate modeling of the failure-time distributions for failures from different sources. This topic arose at the workshop in several somewhat unrelated contexts. First, IRGS considers separately fault modes that are characteristic of a mature component and those that are characteristics of a component still capable of further development, referred to as type A and B failure modes. Second, Frank Camm mentioned that failures in the field are overrepresented by poorly produced components, referred to as “bad actors,” possibly resulting from a poorly controlled manufacturing process. Third, Bill Meeker noted that some failures are unpredictable (possibly because of changes in the manufacturing process) and therefore in need of separate modeling, which he referred to as “special-cause failures” as distinguished from “common-cause” failures.

Two related notions—the separation of a system’s components into those that are mature and immature and the separation of the results of an industrial process into those systems that are indicative of the proper and improper functioning of the process—arose in these and other parts of the workshop. This separation of failures due to components or processes that are or are not functioning as intended is clearly worth greater investigation for its applicability to reliability analysis for defense systems.

There are problems involved in making this idea operational, that is, in designating which failures are due to mature or immature components, and

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