An electrical component has a time-to-failure (or lifetime) distribution that is exponential with param- eter , so the mean lifetime is = 1/A. Suppose that a sample of n of these components is put on test, and let X, be the observed lifetime of component. The test con- tinues only until the rth unit fails, where r 1001-% confidence interval for mean lifetime =1/A.

(b) Suppose 20 units were put on test, and the test terminated after 10 failures occurred. The failure times (in hours) are 15, 18, 19, 20, 21, 21, 22, 27, 28, and 29.

Find a 95% confidence interval on mean lifetime.