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Practical application of Bayes' formulas
World Abstracts on Microelectronics and Reliability manufacturer or technology level. As a consequence of their development, this paper describes the use of life cycle cost modelling in the design and evaluation of competing SEM/ non-SEM modular systems.
Practical application of Bayes' formulas. J. G. MALCOLM, Proc. A. Reliab. Maintainab. Syrup. 180 (1983). Five practical applications of Bayes' formulas are presented. One of these deals with the problem that the military has been wrestling with for years, namely the excessive number of units checking no fault in the intermediate level shop. This problem is variously known as the RTOK problem ("retest OK") or the BCS problem ("bench check serviceable") or the CND problem ("cannot duplicate"). Traditionally this problem is attacked as though it represents a deficiency in the test that was used to cause the unit to be removed and replaced in the system, typically built-in-test (BIT). This paper demonstrates that the RTOK rate can be a function of the prevalence of faulty systems. Throughout the paper, the duality between medical diagnostics and engineering diagnostics is stressed. This is done in order to encourage engineers to consider the applicability of medical diagnostic techniques to their field. Both fields must cope with false alarms and missed faults. Statistical guidelines are presented in the paper indicating how designers can develop tests which will provide an optimal balance between false alarms and missed faults. Ridge detection of culprit variables. JEAN MIRRA, FRANK MCNOLTY, ELDON HANSEN and MARIANBRUSATO.Proc. A. Reliab. Maintainab. Symp. 212 (1983). Rank order stability problems which occur in the analysis of highly correlated data sets are investigated. The Hoerl-Kennard ridge method of biased estimation is adapted to the problem of rank ordering a set of manufacturing and aging variables according to their significance in influencing the success or failure of tested components. Real-world examples are presented which demonstrate the usefulness of the ridge estimator as a diagnostic tool and as a method for achieving stability as the sample size and/or number of variables change. The ridge method consistently provides more stable and physically meaningful results than ordinary linear discriminant techniques. Reliability of shuttle mission control center software. MARTIN L. SHDOMAN and GEORGE RICI-IESON. Proc. A. Reliab. Maintainab. Syrup. 125 (1983). This paper presents the results of a study made of the reliability of software for the Space Shuttle Mission Control Center Data Processing Complex. The ground based software, which is approximately 1.2 million lines of source code, was used to simulate the mission prior to flight, for use in flight controller and astronaut training. During the course of the simulation, all discrepancies from correct behaviour were reported, and subsequently diagnosed as due to hardware, software, or operator errors. The model predictions are compared with the performance of this software during the first shuttle flight. The software was modelled using the exponential software reliability model (Shooman, 1972), to predict the number of errors remaining in the software, the failure rate during the simulated mission, and the mean time between failures. The predicted failure rate was combined with the Poission probability distribution to predict the expected number of software failures during the flight support period of 129 hours. The time period during the shuttle flight (which was 54.5 hr) was also addressed (see Richeson, Jan. 1981 ). The results of the model predictions were then compared to the actual number of software errors experienced during the mission support period, and compared well. Thus, the early phases of simulation helped determine a growth type model which helped predict the reliability of the software at
173
later stages in the continuing simulation process, as well as during operation.
RAM analysis requirements for avionic systems. ALBERTOB. CALVO and PAUL BABCOCK. Proc. A. Reliab. Maintainab. Syrup. 205 (1983). This paper presents a recent application of a Reliability/Availability/Maintainability (RAM) analysis framework for avionic systems that undergo continual modifications. The framework, as defined, consists of a data base and analytic models. The data base uses existing Air Force Logistics Command (AFLC) data systems, base level maintenance information, data on design/manufacturing changes introduced at the factory and depot repair data. Analytic models used in the RAM analysis framework include PREP (Production Reliability Evaluation and Projection) which generates field reliability projections given design/manufacturing changes introduced at the factory and SOAR (Simulation of Operational Availability/Readiness) which provides equipment readiness evaluations under combat employment conditions. Preliminary results are discussed on the application of the RAM analysis framework to an avionic system in its initial operational years. Conclusions regarding RAM analysis requirements for avionics are drawn. Maintenance centered reliability. Roy L. HORN and FRED M. HALL. Proc. A. Reliab. Maintainab. Syrup. 197 (1983). Historically, many studies, analyses, and experiments have been conducted to determine reasons for the differences between the reliability that is experienced in field use and that which was anticipated during the design stage. Most of the investigations have determined that observed differences in reliability are due to increased stress levels induced by the field environment, such as vibration and on-off cycling. As a result, environmental factors are now generally considered in the design process. However, the maintenance process, a possible major contributor to field failures, has not been addressed in the same depth. This paper uses supplemental analytical techniques and data from previous studies to develop the logic for concluding that the maintenance process is a major contributor to unreliability experienced in the field. Techniques for estimating the extent of the maintenance contribution are described, as well as techniques that could be used to help identify maintenance induced failures and their causes. A brief review of the state-of-the-art of maintainability engineering is provided, with recommendation of techniques useful in "hands on" maintainability design engineering. Repair/reliability guarantee programs can work. EDWARD A. RYPKA and GEORGE F. KUJAWSKI.Proc. A. Reliab. Maintainab. Symp. 221 (1983). Repair and reliability guarantee programs have been in existence for some years, however it is only recently that data has been gathered on military type programs that can be measured. This paper reviews an actual case of a Repair/Reliability Guarantee Program that was specifically designed for a unique program. The derivation of some of the growth factors, the incentives, penalties and major contract points are also reviewed. Some of the considerations and techniques that were employed to successfully conclude this program are also discussed, The conclusions and results of the program are displayed. This program was successfully concluded because all parties involved worked together toward that goal. Many of the intangible items that create a workable program cannot be written in contractual language, the key to success is still the people involved and the degree of understanding between customer and contractor. Availability considerations for satellite links. FRANCOIS F. BEHMANN and GEORGE Y. NAWAR. Proc. A. Reliab. Maintainab. Syrup. 269 (1983). The achieved availability of satellite links is evaluated in comparison with the original availability predictions.
Practical application of Bayes' formulas. J. G. MALCOLM, Proc. A. Reliab. Maintainab. Syrup. 180 (1983). Five practical applications of Bayes' formulas are presented. One of these deals with the problem that the military has been wrestling with for years, namely the excessive number of units checking no fault in the intermediate level shop. This problem is variously known as the RTOK problem ("retest OK") or the BCS problem ("bench check serviceable") or the CND problem ("cannot duplicate"). Traditionally this problem is attacked as though it represents a deficiency in the test that was used to cause the unit to be removed and replaced in the system, typically built-in-test (BIT). This paper demonstrates that the RTOK rate can be a function of the prevalence of faulty systems. Throughout the paper, the duality between medical diagnostics and engineering diagnostics is stressed. This is done in order to encourage engineers to consider the applicability of medical diagnostic techniques to their field. Both fields must cope with false alarms and missed faults. Statistical guidelines are presented in the paper indicating how designers can develop tests which will provide an optimal balance between false alarms and missed faults. Ridge detection of culprit variables. JEAN MIRRA, FRANK MCNOLTY, ELDON HANSEN and MARIANBRUSATO.Proc. A. Reliab. Maintainab. Symp. 212 (1983). Rank order stability problems which occur in the analysis of highly correlated data sets are investigated. The Hoerl-Kennard ridge method of biased estimation is adapted to the problem of rank ordering a set of manufacturing and aging variables according to their significance in influencing the success or failure of tested components. Real-world examples are presented which demonstrate the usefulness of the ridge estimator as a diagnostic tool and as a method for achieving stability as the sample size and/or number of variables change. The ridge method consistently provides more stable and physically meaningful results than ordinary linear discriminant techniques. Reliability of shuttle mission control center software. MARTIN L. SHDOMAN and GEORGE RICI-IESON. Proc. A. Reliab. Maintainab. Syrup. 125 (1983). This paper presents the results of a study made of the reliability of software for the Space Shuttle Mission Control Center Data Processing Complex. The ground based software, which is approximately 1.2 million lines of source code, was used to simulate the mission prior to flight, for use in flight controller and astronaut training. During the course of the simulation, all discrepancies from correct behaviour were reported, and subsequently diagnosed as due to hardware, software, or operator errors. The model predictions are compared with the performance of this software during the first shuttle flight. The software was modelled using the exponential software reliability model (Shooman, 1972), to predict the number of errors remaining in the software, the failure rate during the simulated mission, and the mean time between failures. The predicted failure rate was combined with the Poission probability distribution to predict the expected number of software failures during the flight support period of 129 hours. The time period during the shuttle flight (which was 54.5 hr) was also addressed (see Richeson, Jan. 1981 ). The results of the model predictions were then compared to the actual number of software errors experienced during the mission support period, and compared well. Thus, the early phases of simulation helped determine a growth type model which helped predict the reliability of the software at
173
later stages in the continuing simulation process, as well as during operation.
RAM analysis requirements for avionic systems. ALBERTOB. CALVO and PAUL BABCOCK. Proc. A. Reliab. Maintainab. Syrup. 205 (1983). This paper presents a recent application of a Reliability/Availability/Maintainability (RAM) analysis framework for avionic systems that undergo continual modifications. The framework, as defined, consists of a data base and analytic models. The data base uses existing Air Force Logistics Command (AFLC) data systems, base level maintenance information, data on design/manufacturing changes introduced at the factory and depot repair data. Analytic models used in the RAM analysis framework include PREP (Production Reliability Evaluation and Projection) which generates field reliability projections given design/manufacturing changes introduced at the factory and SOAR (Simulation of Operational Availability/Readiness) which provides equipment readiness evaluations under combat employment conditions. Preliminary results are discussed on the application of the RAM analysis framework to an avionic system in its initial operational years. Conclusions regarding RAM analysis requirements for avionics are drawn. Maintenance centered reliability. Roy L. HORN and FRED M. HALL. Proc. A. Reliab. Maintainab. Syrup. 197 (1983). Historically, many studies, analyses, and experiments have been conducted to determine reasons for the differences between the reliability that is experienced in field use and that which was anticipated during the design stage. Most of the investigations have determined that observed differences in reliability are due to increased stress levels induced by the field environment, such as vibration and on-off cycling. As a result, environmental factors are now generally considered in the design process. However, the maintenance process, a possible major contributor to field failures, has not been addressed in the same depth. This paper uses supplemental analytical techniques and data from previous studies to develop the logic for concluding that the maintenance process is a major contributor to unreliability experienced in the field. Techniques for estimating the extent of the maintenance contribution are described, as well as techniques that could be used to help identify maintenance induced failures and their causes. A brief review of the state-of-the-art of maintainability engineering is provided, with recommendation of techniques useful in "hands on" maintainability design engineering. Repair/reliability guarantee programs can work. EDWARD A. RYPKA and GEORGE F. KUJAWSKI.Proc. A. Reliab. Maintainab. Symp. 221 (1983). Repair and reliability guarantee programs have been in existence for some years, however it is only recently that data has been gathered on military type programs that can be measured. This paper reviews an actual case of a Repair/Reliability Guarantee Program that was specifically designed for a unique program. The derivation of some of the growth factors, the incentives, penalties and major contract points are also reviewed. Some of the considerations and techniques that were employed to successfully conclude this program are also discussed, The conclusions and results of the program are displayed. This program was successfully concluded because all parties involved worked together toward that goal. Many of the intangible items that create a workable program cannot be written in contractual language, the key to success is still the people involved and the degree of understanding between customer and contractor. Availability considerations for satellite links. FRANCOIS F. BEHMANN and GEORGE Y. NAWAR. Proc. A. Reliab. Maintainab. Syrup. 269 (1983). The achieved availability of satellite links is evaluated in comparison with the original availability predictions.