SRC Forum - Message Replies
Forum: Reliability & Maintainability Questions and Answers
Topic: Reliability & Maintainability Questions and Answers
Topic Posted by: Reliability & Maintainability Forum
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998
Posted by: Ron
Date posted: Tue Apr 10 16:19:42 US/Eastern 2001
Subject: Confidence level of the prediction
Always can learn a lot from your guys. Here is the question. I was told that the confidence level for 217 prediction is 60%, and for Bellcore is 90%. Is it true? I checked with 217 HDBK, no such number is found. Only found "the failure rate models are point estimates".
BTW, what's the confidence level for PRISM? Thanks in advance.
Subject: Confidence Levels
Reply Posted by: Bruce Dudley
Organization: Reliability Analysis Center
Date Posted: Wed Apr 18 13:19:28 US/Eastern 2001
Part failure models in MIL-HDBK-217 and Telcordia reliability models are based on part data from numerous sources. Complete models are hardly ever developed under a single study contract nor are the failure data from a single source. For example, all MIL-HDBK-217 environmental factors were developed under separate study efforts from the part failure models. Therefore, because of the fragmented nature of the data and the fact that it is often necessary to interpolate or extrapolate from many available data sources when developing new models, no statistical confidence intervals should be associated with the overall model results. In addition, adding vendor and field failure rate data to the combination, results in a mixed prediction that may or may not represent the "new" design. Outside data sources are usually from units or components that have been previously developed and can be similar to the new but may have different technologies, hence, indeterminable correlation to the "new" design.
If the similar units or components are representative of the technology of the "new" design and sufficient hours and failures are available, confidence levels can be developed using the CHI SQUARE statistics for those "elements". Combining those elements with MIL-HDBK-217 prediction models is not possible as the MIL-HDBK-217 models do not have add-on data capabilities built-in, so complete confidence limit assessment is lacking until actual confidence level testing can be performed on the entire product.
For your information, our new PRISM reliability model does have some built-in capability of adding test and field data to change the model outputs. Inaddition, induced failures, software problems and overstress conditions are included in the PRISM reliability model, but confidence levels are not included.