Example of creating a demonstration test plan
main topic     interpreting results     session command     see also 

The reliability goal for a turbine engine combustor is a 1 percentile of at least 2000 cycles. The number of cycles to failure tends to follow a Weibull distribution with shape = 3. You can accumulate up to 8000 test cycles on each combustor. You must determine the number of combustors needed to demonstrate the reliability goal using a 1-failure test plan.

1    Choose Stat > Reliability/Survival > Test Plans > Demonstration.

2    Choose Percentile, then enter 2000. In Percent, enter 1.

3    In Maximum number of failures allowed, enter 1.

4    Choose Testing times for each unit, then enter 8000.

5    From Distribution, choose Weibull. In Shape (Weibull) or scale (other dists), enter 3. Click OK.

Session window output

Graph window output

Interpreting the results

You must test 8 combustors for 8000 cycles to demonstrate with 95.2% confidence that the first percentile is at least 2000 cycles.

The graph shows the likelihood of actually passing the test that you specified. Here,

·    The probability that your 1-failure test will pass increases steadily as the improvement ratio increases from zero to two.

·    If the improvement ratio is greater than about two, the test has an almost certain chance of passing.

·    If the (unknown) true first percentile was 4000, then the improvement ratio = 4000/2000 = 2, and the probability of passing the test would be about 0.88. If you reduced the value to be demonstrated to 1600, then the improvement ratio would increase to 2.5 and the probability of passing the test would increase to around 0.96. By reducing the value to be demonstrated, you would increase the probability of passing the test. However, you would also be making a less powerful statement about the reliability of the turbine engine combustor.