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===Classifications===
#REDIRECT [[Crow_Extended_-_Continuous_Evaluation#Classifications]]
Under the Crow Extended - Continuous Evaluation model, corrective actions can be fixed at the time of failure or delayed to a later time (before time  <math>T</math> , at time  <math>T</math>  or after time  <math>T</math> , where  <math>T</math>  indicates the test's end time). The definition of delayed is expanded to include all Type B failure modes corrected after the time of failure. This will include most, if not all, design-related failure modes requiring a root cause failure analysis. Failure modes that are corrected at the time of failure are typically related to human factors, such as manufacturing, operator error, etc.
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For the Crow Extended - Continuous Evaluation model, the classifications are defined as follows:
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:• A indicates that a corrective action will not be performed (management chooses not to address these modes for technical, financial or other reasons).
:• BC is defined as a failure mode that receives a corrective action at the time of failure and before the testing resumes. Typically a type BC failure mode does not require extensive root cause failure analysis and therefore can be corrected quickly. Type BC modes are generally easy to fix and are usually related to issues such as quality, manufacturing, operator, etc.
:• BD is defined as a failure mode that receives a corrective action at a test time after the first occurrence of the failure mode. Therefore, a fix is considered delayed if it is not implemented at the time of failure. A delayed fix can then occur at a later time during the test, at the end of the test (before the next phase) or during another test phase. Type BD failure modes typically require failure analysis and time to fabricate the corrective action. During the period  <math>(0,T)</math>  into the test, there may be BD modes with corrective actions incorporated into the systems and other BD modes that have been seen but not yet fixed.
Table 10.1 shows a comparison between the definitions of the classifications in the Crow Extended and Crow Extended - Continuous Evaluation models:
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<center>Table 10.1 - Comparison of classification definitions</center>
<math>\begin{matrix}
  Classification & Crow Extended & Crow Extended -  \\
  {} & {} & Continuous Evaluation  \\
  A & \text{  Corrective action will not be performed}\text{.} & \text{  Same as Crow Extended}\text{.}  \\
  BC & \text{  Corrective action during the test}\text{.} & \text{  Corrective action at the time of failure}\text{.}  \\
  BD & \begin{matrix}
  \text{Corrective action delayed until }  \\
  \text{after the completion of the test}\text{.}  \\
\end{matrix} & \begin{array}{*{35}{l}}
  \text{Corrective action delayed to a test time }  \\
  \text{after the first occurrence of the failure mode}\text{.}  \\
\end{array}  \\
\end{matrix}</math>
 
Reliability growth is achieved by decreasing the failure intensity. The failure intensity for the A failure modes will not change. Therefore, reliability growth can be achieved only by decreasing the BC and BD mode failure intensities. It is also clear that, in general, the only part of the BD mode failure intensity that can be decreased is the part that has been seen during testing. BC failure modes and fixed BD modes (delayed fixes implemented during the test) are corrected during testing and their failure intensities will not change any more at the end of test.
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Latest revision as of 02:34, 24 August 2012

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