Reliability HotWire

Issue 94, December 2008

Tool Tips

* Can I compare analysis results using different settings in DOE++?

Beginning with version 1.0.2, DOE++ offers the Analysis History window, which allows you to view the results for previous analyses of a selected response. To access the history, select a response in the Analysis tab Control Panel in a Standard Folio or Multiple Linear Regression Tool then choose Data > View History or click the Analysis History icon.

The Analysis History window is shown next.

Each time you make a change to the analysis settings and perform an analysis, DOE++ saves a copy of the contents of the Analysis tab, along with the alias structure. The list of saved results is presented in a hierarchical tree structure on the left side of the window. The response name is displayed at the top of the tree structure and the historical sets of results, from newest to oldest, are displayed below. Click an item to display the results in the spreadsheet area on the right.

You can apply the analysis settings from the currently selected historical results to the Analysis tab by clicking the Apply Settings button in the toolbar then clicking the Calculate icon the Main page of the Analysis tab Control Panel.

This will include the alpha value, distribution, transformation, type of sum of squares, test terms and selected effects.

* What are the Severity Class and Failure Probability ratings scales for in Xfmea/RCM++?

A typical FMEA incorporates some method to identify and evaluate the risk associated with the potential problems identified through the analysis. Xfmea and RCM++ support both Risk Priority Numbers (RPNs) and Criticality Analysis.

RPNs: To use the Risk Priority Number (RPN) method to assess risk, the analysis team must:

  • Rate the severity of each effect of failure.
  • Rate the likelihood of occurrence for each cause of failure.
  • Rate the likelihood of detection for each cause of failure.
  • Calculate the RPN by obtaining the product of the three ratings:

RPN = Severity x Occurrence x Detection

Criticality Analysis: To use the quantitative Criticality Analysis method, the analysis team must:

  • Define the reliability/unreliability for each item and use this to estimate the expected number of failures at a given operating time.
  • Identify the portion of the items unreliability that can be attributed to each potential failure mode.
  • Rate the probability of loss (or severity) that will result from each failure mode that may occur.
  • Calculate the criticality for each potential failure mode by obtaining the product of the three factors:

Mode Criticality = Expected Failures x Mode Ratio of Unreliability x Probability of Loss

  • Calculate the criticality for each item by obtaining the sum of the criticalities for each failure mode that has been identified for the item.

Item Criticality = SUM of Mode Criticalities

To use the qualitative Criticality Analysis method, the analysis team must:

  • Rate the severity of the potential effects of failure.
  • Rate the likelihood of occurrence for each potential failure mode.
  • Compare failure modes via a Criticality Matrix, which identifies severity on the horizontal axis and occurrence on the vertical axis.

You can see that the processes for RPNs and for qualitative Criticality Analysis both involve rating the severity of the effect and rating the likelihood of occurrence. Although most practitioners tend to use one risk assessment method or the other, some organizations may choose to employ both analysis approaches in order to satisfy customer requirements or for other reasons. Therefore, beginning with Xfmea/RCM++ version 4.0.1, you can assign separate rating scales in the same project for the "Severity" rating used to calculate an RPN and the "Severity Class" used for Criticality Analysis; likewise for the "Occurrence" rating used to calculate an RPN and the "Failure Probability" used for Criticality Analysis. These scales are managed via the Severity, Occurrence, Severity Class and Failure Probability pages of the Project Properties window and via the Severity and Occurrence pages of the Profiles/Libraries Manager.

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