Example 3 - Developmental Testing with Corrective Actions Implemented During the Test and Delayed Fixes

Examples provided here are purely for illustrating software features and functionality.

Example 3 – Developmental Testing with Corrective Actions Implemented During the Test and Delayed Fixes

Software Used: Software Used: ALTA PRO

Download Example File for Version 10 (*.rsgz10) or Version 9 (*.rsr9)

This example is based on the paper An Extended Reliability Growth Model For Managing And Accessing Corrective Actions by Dr. Larry Crow, presented at the 2004 RAMS. [Click here to download the paper (*.pdf, 238 KB)]

When a system is tested and failure modes are observed, management can decide whether or not to fix the failure mode. Therefore, the management strategy places failure modes into two categories: A modes and B modes. A modes are all failure modes that are discovered during the test but will not be fixed. This accounts for all modes which management determines are not justified to fix for economic or other reasons. In order to provide the assessment and management metric structure for corrective actions during and after a test, two types of B modes are defined. BC modes are corrected during the test, and the corrective actions for BD modes are delayed until the end of the test. The management strategy is defined by how the corrective actions, if any, will be implemented. In summary, the classifications are defined as follows:

  • A indicates that no fix will be applied.
  • BC indicates that a fix was applied during the test.
  • BD indicates that the fix will be delayed until after the test.

The Crow Extended model is used to analyze data that includes these failure classifications.

Background

A product undergoes 400 hours of developmental testing. During testing, the observed failure modes are identified. The data set is given in the next three tables, where the Failure Mode column combines the mode classification with the mode’s numerical identifier. (Identifiers were not assigned to A modes since they will not be addressed.)

Failure Time Failure Mode   Failure Time Failure Mode   Failure Time Failure Mode
0.7 BC17 121.9 BC22 285 BD13
3.7 BC17 125.5 BD9 304 BD9
13.2 BC17 133.4 BD10 315.4 BD4
15 BD1 151 BC23 317.1 A
17.6 BC18 163 BC24 320.6 A
25.3 BD2 164.7 BD9 324.5 BD12
47.5 BD3 174.5 BC25 324.9 BD10
54 BD4 177.4 BD10 342 BD5
54.5 BC19 191.6 BC26 350.2 BD3
56.4 BD5 192.7 BD11 355.2 BC26
63.6 A 213 A 364.6 BD10
72.2 BD5 244.8 A 364.9 A
99.2 BC20 249 BD12 366.3 BD2
99.6 BD6 250.8 A 373 BD8
100.3 BD7 260.1 BD1 379.4 BD14
102.5 A 263.5 BD8 389 BD15
112 BD8 273.1 A 394.9 A
112.2 BC21 274.7 BD6 395.2 BD16
120.9 BD2 282.8 BC17

An effectiveness factor based on engineering assessments is also assigned to the BD modes. This factor is the expected fractional decrease in failure intensity of a failure mode after the implementation of a corrective action (e.g., 0.7 means 70% of the mode’s failure intensity will be removed by the fix). The factors for each BD mode are shown next.

BD Mode Effectiveness
Factor
1 0.7
2 0.7
3 0.8
4 0.8
5 0.9
6 0.9
7 0.5
8 0.9
9 0.9
10 0.7
11 0.7
12 0.6
13 0.6
14 0.7
15 0.7
16 0.5

Analysis and Discussion

The Crow Extended model uses the maximum likelihood estimation (MLE) method to calculate the parameters, which is known to produce a biased value for beta. If the software is not configured to remove this bias, the option to remove it should be selected in the Application Setup.

Figure 1: Application Setup window showing that the the bias will be removed from beta.

A standard folio data sheet configured for exact failure times is created by selecting Times-to-Failure Data > Failure Times in the RGA Folio Data Sheet Setup window.

Figure 2: Selecting the data type for the new folio.

The Crow Extended model is selected and the data set is entered.
An effectiveness factor is assigned to each BD failure mode.

Figure 4: Effectiveness Factor window with the specified factors for each BD mode.

Finally, it is specified that the tested ended at 400 hours. (If it were “failure terminated” it would end at the time of the last failure, which in this case is 395.2 hours.)

Figure 5: Termination Time window showing a time of 400.

After analyzing the data, a summary of results on the control panel shows the parameters of the fitted model.

Results and Discussion

The summary of results shows that the demonstrated MTBF (DMTBF) for this system at the end of the test is 7.8471 hours, which is the result of the corrective action taken during the test (BC modes).

If the 16 delayed corrective actions are implemented (BD modes), the MTBF is projected to be 11.3182 hours. This can be calculated using the Quick Calculation Pad (QCP).

Figure 5: QCP showing the expected MTBF after BD modes are fixed.

If testing continues with the current management strategy in place (i.e., modes corrected vs. modes not corrected) and with the current effectiveness of each corrective action, then the maximum attainable MTBF is about 15 hours. This is called the growth potential MTBF. The following plot illustrates shows the projected MTBF (red), growth potential MTBF (red) and the demonstrated MTBF (blue).

Figure 6: Growth Potential MTBF plot showing the demonstrated, projected and growth potential MTBFs.

The management strategy is summarized in the Failure Mode Strategy chart.