How to Design Reliability Tests
Below is a step by step example of how to reduce reliability demonstration testing requirements by over 50%.  The example utilizes bayesian testing module of the Reliability & Maintenance Analyst softwareClick here to download a free demo version of this software or click here for the web based module.  The bayesian testing module is fully functional in the demo version!

For a related article in Adobe Acrobat format with all the technical details of  the example below click here.  For the same article in Microsoft Word for Windows 95 format click here.

A new product is required to demonstrate 99% reliability at 100 million cycles with 80% confidence.  Given a Weibull shape parameter of 2.8, how many units must be tested for 100 million cycles to demonstrate the required reliability assuming no failures?

### Solution

1. Select "Testing" from the menu, and the following screen appears.

2. Enter the shape parameter of 2.8, a confidence level of 0.8, reliability of 0.99, a time of 100, and a test duration of 100.  Click the "Sample Size" radio button, and the sample size will be computed.  To demonstrate 99% reliability with 80% confidence at 100 million cycles requires testing 161 units for 100 million cycles with no failures.  This is shown in the figure below.

3. Change the Test Duration to 200, and click the "Sample Size" radio button.   To demonstrate 99% reliability with 80% confidence at 100 million cycles requires testing 23 units for 100 million cycles with no failures.  This is shown in the figure below.  By testing a sample size of 23 for 200 million cycles ( a total of 4600 million test cycles) instead of 161 samples for 100 million cycles ( a total of 16,100 million test cycles), the total number of test cycles is reduces by 71.4%.  If the shape parameter is greater than 1, the total number of test cycles decreases as the test duration increases.

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