Reliability HotWire

Reliability HotWire

Issue 128, October 2011

Tool Tips

*How should I use the Grouped Data Settings in Weibull++?

The grouped data settings in Weibull++ affect how the unreliability estimates are plotted on the probability plot. These settings are available only when working with grouped data.

When using rank regression with grouped data, the software will plot only the data points that correspond to the highest median rank position in each group. For example, consider two groups of data with 10 units each. By default, the software will plot only two data points: the 10th rank position out of 20, and the 20th rank position out of 20. The regression line is then fitted to the two points. When you select the Ungroup on Regression option, the software will plot each individual data point in the group. The regression line is then fitted to all the data points. In general, grouped data analysis provides parameter estimates that have wider confidence bounds, while ungrouped data analysis provides parameter estimates with much narrower confidence bounds.

When using MLE analysis, the plotted line is independent of whether data points are treated as grouped or ungrouped data. The option to Ungroup on MLE Plot is only for changing the way the data points are displayed on the MLE probability plot.

Note: When working with grouped failure data, you have to be cautious about ungrouping the data. In grouped data, it is assumed that the failures occur at some time in the interval between the previous and the current time-to-failure. For example, for a group of 10 units with a failure time of 100 the software will assume that the 10 units failed between 0 and 100 hours. If you select the option to Ungroup on Regression, the software will treat the 10 units as failures that occur at exactly 100 hours.

Grouped Data Settings in Control Panel

*How can I customize the way Xfmea/RCM++ prioritizes and highlights failure causes listed in my FMEA?

In order to highlight the most serious issues in the design you’re analyzing, the Highlight Priority feature in Xfmea/RCM++ allows you to color-code the failure causes listed in your FMEA. By default, when the Highlight Priority feature is enabled, the software uses a simple risk ranking logic to assign failure causes to three priority levels (which are named "Low," "Medium" and "High," and color-coded in green, yellow and red, respectively). This simple logic allows you to set the thresholds for each level based on any one of the risk priority metrics (e.g., RPN, Severity, etc.) that are enabled for the project.

As an alternative, you can use the Risk Ranking feature (available in Version 5.0.1 or higher) to customize the number of priority levels, as well as their associated names and colors. This feature also allows you to define a more complex ranking logic that considers multiple metrics rather than, say, RPN alone. This configurable logic can be saved in the active library so it will be easy to apply to any particular analysis project.

To save customized priority levels and their associated risk ranking logic in the active library, open the Profiles/Libraries Manager and select an interface style that will include your customized logic. In the Edit Interface Style window, click RPNs under the Risk Assessment heading in the navigation panel. Then, in the Highlight Priority Based on area, choose Risk Ranking from the drop-down list and click the Risk Ranking Logic button.

Edit Interface Style Window

In the window that appears, click the icon inside the Priority column heading to open the tool for defining priority levels.

Defining Priority Levels

To create a new priority level, enter the appropriate information into an empty row. (Xfmea will always include an empty row for defining new priority levels. If you delete the empty row, it will reappear when you close and reopen the window.) Return to the Risk Ranking window when you are finished defining your priority levels.

Every row in the Risk Ranking window represents a rule for assigning causes to priority levels. If a cause satisfies all the criteria in a row, it will be assigned the priority level displayed at the end of that row, and it will be color-coded with whatever color you selected for that priority level. You can define up to three criteria for each rule, and each criterion has three columns. The first column specifies a rating or metric while the third column specifies a numerical value. The middle column specifies whether the rating/metric must be equal to (=), greater than (>), less than (<), greater than or equal to (>=) or less than or equal to (<=) the specified numerical value.

As an example, suppose you decided to include the following risk ranking logic in your interface style.

  • A failure cause has a CRITICAL priority level if its severity rating is 9 or higher AND its RPN is 300 or higher.
  • A failure cause has a HIGH priority level if its severity rating is 6 or higher OR its RPN is greater than 200.
  • A failure cause has a MEDIUM priority level if its severity rating is greater than 3 OR its RPN is 100 or higher.
  • All other failure causes have a LOW priority level.

You would configure the Risk Ranking window as shown next.

Risk Ranking Window

Note: The first rules in the table are given priority over later rules. So, for example, if a cause has a severity rating of 10 and an RPN of 800, then it will be marked critical rather than high priority, even though it also satisfies the criteria for high priority.

After saving this interface style, you can apply it to any particular analysis project. To turn the Highlight Priority feature on or off while you’re working with the analysis data, choose FMEA > Highlight Priority or click the Highlight Priority icon.

Highlight Priority Icon

When the Highlight Priority feature is turned on, the failure causes will be color-coded according to the ranking logic defined in your interface style. The color-coding will appear in the worksheet and hierarchy views of your FMEA, and it will also appear in the filtered view when filtered by causes.