AMSAA maturity projection model based on Stein estimation

Assessment methods are presented for projecting the impact of corrective actions, or fixes, on system reliability. The presented approach to reliability projection, referred to as AMPM-Stein, addresses the case where all fixes are delayed to the end of the current test phase. The assessment procedure allows for a unified treatment of failure modes. The procedure when applied in this fashion only distinguishes for estimation purposes between modes for which a fix would be attempted if surfaced (B-modes) and those that would not be addressed with a fix even if surfaced (A-modes) through assigned positive and zero fix effectiveness factors (FEFs) for surfaced B-modes and A-modes, respectively. A projection procedure that can treat failure modes in such a unified manner is necessary in cases where it is not realistic to divide the potential failure modes into an inherent set of A-modes and B-modes. In many instances A-modes are not inherent. Modes could be reclassified from A to B due to repeat occurrences and the necessity of meeting a reliability requirement, additional information, or changes in the level of resources available for corrective action. The AMPM-Stein procedure can also be applied to the case addressed by widely used projection methods for which there is an assumed inherent division between potential failure modes with respect to A-mode/B-mode categorization. Current simulation results indicate that greater accuracy with regard to the MTBF estimates can be achieved by the AMPM-Stein approach even when such an A-mode/B-mode split is valid. In particular, these simulation results indicate that the AMPM-Stein MTBF projections obtained for this case tend to be more accurate than those obtained from the reliability projection method adopted as a standard by the International Electrotechnical Commission. Also the simulations indicate that the AMPM-Stein MTBF assessments based on simple closed form parameter estimators obtained from the method of moments are almost as accurate as the AMPM-Stein assessments that utilize the more computationally intensive maximum likelihood estimators. The AMPM-Stein approach has several additional appealing characteristics. In contrast to current projection methods, the approach precludes the need to assess the arithmetic average of the fix effectiveness factor values that would be realized if all B-modes were to be surfaced. It only requires assessment of FEFs for the surfaced modes that will be mitigated. In addition, the AMPM-Stein approach naturally leads to an expression for the portion of the mitigated system failure rate due to the unsurfaced modes (or unsurfaced B-modes for two classifications). Thus no functional form for this failure rate need be assumed.