Nevertheless, academic and industrial research on reliability methods has continued, and substantial progress has been made in the last 20–25 years. Areas of recent progress include: (1) methods for combining information across test environments, including methods for incorporating subjective assessments; (2) fatigue modeling; (3) statistical methods for software engineering; (4) nonparametric or distribution-free methods, specifically for reliability growth, but also more generally (an important example being for variance estimation); (5) alternative methods for modeling reliability growth; (6) treatment of censored or missing data; (7) use of accelerated testing methods; and (8) greater use of physics-of-failure models and procedures that are helpful in identifying the primary sources of failure.

Chapter 3 presented an argument expressed by several workshop speakers: that DoD needs not only to upgrade the “tried and true” reliability methods that could be disseminated in a handbook, but also to stay abreast of methods on which current research is being carried out or for which the full extent of the applicability of recent methods to defense systems is still unclear. Application of these methods may still require greater resources, but many of them are likely to provide important, substantial advantages over current methods. The issue is how the test service agencies and other members of the test and evaluation community can gain easier access to contemporary reliability methods. As discussed at the workshop, one important way to address this issue would be to identify properties of a reference book that could be made available to help provide this linkage between the defense and statistical communities. The primary means suggested for accomplishing this was updating or redesigning the RAM Primer.

PHYSICS-OF-FAILURE MODELS AND METHODS FOR SEPARATELY MODELING FAILURES FROM DIFFERENT SOURCES

While no session was devoted specifically to either greater use of physics-of-failure models (i.e., models that directly represent the physical basis for failure) in modeling reliability for defense systems or methods for separately modeling failures due to distinct sources, these two related topics arose repeatedly during various workshop sessions. Several speakers supported the greater use of physics-of-failure models whenever possible to acquire a better understanding of the sources and effects of component and



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