# Repairable System Test Design Example - Solve for Sample Size

*This example appears in the Reliability Growth and Repairable System Analysis Reference*.

At the end of a reliability growth testing program, a manufacturer wants to demonstrate that a new product has achieved an MTBF of 10,000 hours with 80% confidence. The available time for the demonstration test is 4,000 hours for each test unit. Assuming zero failures are allowed, what is the required number of units to be tested in order to demonstrate the desired MTBF?

**Solution**

We can obtain the required number of units by using the Design of Reliability Tests (DRT) tool in the RGA software. Since this is a demonstration test then [math]\beta =1\,\![/math], and no growth will be achieved. The results are shown next. It can be seen that in order to demonstrate a 10,000 hours MTBF with 80% confidence, 5 test units will be required.

The next figure shows the different combinations of test time per unit, and the number of units in the test for different numbers of allowable failures. It helps to visually examine other possible test scenarios.