Enter your data for Estimation Test Plan

Stat > Reliability/Survival > Test Plans > Estimation
Enter planning information to determine the number of test units that you need to estimate percentiles or reliabilities with a specified degree of precision. Planning information may come from design specifications, expert opinions, small pilot studies, or other sources.
Note

The data you collect for an estimation test can be censored or uncensored. A time-censored test plan or failure-censored test plan often provides precise results while minimizing test costs. For more information, go to Data censoring.

  1. Under Parameter to be Estimated, select an option to indicate what you want to determine.
    • Percentile for percent: Estimate the time at which a certain percentage of the units will fail. This value is useful for determining a warranty period for a product. Enter a value between 0 and 100 to indicate the percentage of units that will fail. The nth percentile has n% of the observations below it, and (100–n)% of observations above it. To see the percent values that correspond with parameters of different distributions, go to Determine sample size requirements for estimating parameters .
    • Reliability at time: Estimate the percentage of units that will survive a certain length of time. This value is useful for estimating how many units will survive through a warranty period. The time could be a standard time measurement, such as number of hours, or another measurement that indicates a certain amount of usage, such as number of cycles, number of weight units, and so on.
  2. Under Precisions as distances from bound of CI to estimate, indicate how you want to define precision for the test.
    1. From the drop-down list, indicate whether you want to specify the precision as a distance from the reliability estimate to either the lower or upper bound of the confidence interval (CI). Selecting the lower bound is common because many reliability standards are defined in terms of the worst-case scenario.
    2. Enter a precision value to indicate the maximum distance from the confidence bound to the estimate. For example, if you want to estimate the 10th percentile of your failure time distribution, and the lower bound is to be no more than 25 hours less than your estimate, then select Lower bound and enter 25.
    Note

    The precision of an estimation test plan is based on the width of the confidence interval for the parameter that you are estimating. A wider confidence interval provides a less precise estimate, but requires fewer units to test.

  3. Specify planning values for two of the following. For example, you could provide planning values for both unknown parameters (the scale and shape or the scale and location). Alternatively, you could provide planning values for two of the percentiles, and have Minitab calculate the value of the unknown parameters. Or, you could provide planning values for one parameter and one percentile.
    • Shape (Weibull) or scale (other distributions): Enter the shape (Weibull) or scale (other distributions). For the exponential distribution, do not enter a value because it does not have a shape parameter. Specify only the scale parameter (below).
    • Scale (Weibull or expo) or location (other dists): Enter the scale (Weibull or exponential) or location (other distributions).
    • Percentile: Enter a percentile that indicates the cut-off value. In Percent: enter a percent associated with the percentile. For example, if you expect 5% of units to fail by 40,000 cycles, enter 40000 in Percentile, and 5 in Percent.
    • Percentile: Enter a percentile that indicates the cut-off value. In Percent: enter a percent associated with the percentile. For example, if you expect 15% of units to fail by 100,000 cycles, enter 100000 in Percentile, and 15 in Percent.
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