Example of creating an accelerated life test plan
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You want to plan an accelerated life test to estimate the 1000-hour reliability of an incandescent light bulb at the design voltage of 110 volts. You have 20 light bulbs available to test until failure. To accelerate failures, you will run the test at 120 volts and 130 volts.

You believe that a power relationship will adequately model the relationship between failure time and voltage. Historical data indicate that a lognormal distribution with a scale of 50 appropriately models light bulb failure. The planning values are 1200 for the 50th percentile at 110 volts and 600 for the 50th percentile at 120 volts.

1    Choose Stat > Reliability/Survival > Test Plans > Accelerated Life Testing.

2    Under Parameter to be Estimated, choose Reliability at time, then enter 1000.

3    In Sample sizes or precisions as distances from bound of CI to estimate, choose Sample size, then enter 20.

4    From Distribution, choose Lognormal. From Relationship, choose Ln (Power).

5    In Shape (Weibull) or scale (other distributions), enter 50.

6    Under Specify planning values for two of the following, do the following:

·    In the first Percentile, enter 1200. In Percent, enter 50. In Stress, enter 110.

·    In the second Percentile, enter 600. In Percent, enter 50. In Stress, enter 120.

7    Click Stresses.

8    In Design stress, enter 110. In Test stresses, enter 120  130. Click OK in each dialog box.

Session window output

Accelerated Life Testing Test Plans

 

 

Uncensored data

Power model

 

Estimated parameter: Reliability at time = 1000

Calculated planning estimate = 0.501455

Design stress value = 110

 

Planning Values

Percentile values = 1200, 600 for percents = 50, 50 at stresses = 110, 120

 

Planning distribution: Lognormal base e

Intercept = 44.5349, Slope = -7.96617 and Scale = 50

 

Selected test plans:  “Optimum” allocations test plans

 

 

Total available sample units = 20

 

 

1st Best “Optimum” Allocations Test Plan

 

  Test    Percent  Percent  Sample  Expected

Stress    Failure    Alloc   Units  Failures

   120        100  65.7524      13        13

   130        100  34.2476       7         7

 

Standard error of the parameter of interest = 0.283150

 

 

2nd Best “Optimum” Allocations Test Plan

 

  Test    Percent  Percent  Sample  Expected

Stress    Failure    Alloc   Units  Failures

   120        100       65      13        13

   130        100       35       7         7

 

Standard error of the parameter of interest = 0.283185

 

 

3rd Best “Optimum” Allocations Test Plan

 

  Test    Percent  Percent  Sample  Expected

Stress    Failure    Alloc   Units  Failures

   120        100       70      14        14

   130        100       30       6         6

 

Standard error of the parameter of interest = 0.284363

Interpreting the results

To estimate the 1000-hour reliability at the design voltage of 110 volts, test 13 units until failure at 120 volts and 7 units until failure at 130 volts.