Enter your data for Demonstration Test Plan

Stat > Reliability/Survival > Test Plans > Demonstration

Enter information to develop plans for a test to demonstrate that only a certain number of failures occur in a set amount of test time. Planning information may come from design specifications, expert opinions, small pilot studies, or other sources.

  1. Under Minimum Value to be Demonstrated, select the criteria to use for the test and then enter the minimum value for passing.
    • Scale (Weibull or expo) or location (other dists): Select to demonstrate the minimum scale for the Weibull and exponential distributions or the minimum location for other distributions. Then enter the scale or location value.
    • Percentile: Select to demonstrate the minimum percentile. In Percentile, enter the percentile. For example, for a test to demonstrate how long a product lasts, enter a value in units of time, such as a number of hours or a number of cycles. In Percent: enter a percent between 0 and 100 for the percentile. For example, to demonstrate reliability, a turbine engine combustor should achieve a 1st percentile of at least 2000 cycles.
    • Reliability: Select to demonstrate the minimum reliability. In Reliability, enter a number between 0 and 1. In Time: enter a time associated with the reliability value.
    • MTTF: Select to demonstrate the mean time to failure (MTTF). Then enter a value for the MTTF. The MTTF is the mean lifetime of an item, which is based on the estimate of the theoretical center of the distribution that accounts for censored observations. For more information, go to What is mean time to failure (MTTF)?
  2. In Maximum number of failures allowed, enter one or more values to indicate the maximum number of failures that your test allows. Entering 0 results in the smallest sample size and test time. However, if just one failure occurs, the demonstration test fails and you cannot conclude that you have achieved the desired reliability. For more information, go to What is the difference between an m-failure test and a 0-failure test?.
  3. Select an option to indicate whether you want to enter the samples sizes or the testing times for the plan. Minitab calculates the value or values for the property that you do not enter.
    • Sample sizes: Enter one or more values to indicate the number of units available for testing.
    • Testing times for each unit: Enter one or more values to indicate the time available for testing.

    Each combination of maximum number of failures allowed and sample size or testing time creates one test plan. You may want to create several test plans by entering multiple values for each property and then compare the results.

  4. Under Distribution Assumptions, from Distribution, select a distribution to model your data. Base your decision on process knowledge or an evaluation of the distribution fit. For more information, go to Distribution fit for reliability analysis.
  5. In Shape (Weibull) or scale (other distributions), enter the shape value for a Weibull distribution or a scale value for another distribution. If you are using the exponential distribution, do not enter a value. Minitab assumes a shape value of 1 for the exponential distribution. When testing a redesigned system, you can often use a historical value to estimate the shape or scale parameter of the distribution. The historical value is usually a good estimate because the redesign does not affect this parameter.

    If your estimate of the parameter is incorrect, then the sample size or the testing time for the test plan may not be accurate. If you are unsure of the parameter value, consider repeating the test plan analysis using a range of likely values for the assumed parameter. The results from the different analyses can help you to determine how sensitive the sample size or testing time is to the assumed parameter.

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