The service life of a battery used in cardiac pace maker is assumed to be normally disturbed. A random sample of ten batteries is subjected to an accelerated life test by running them continuously at an elevated temperature until failure, and the following lifetimes (in hours ) are obtained : 25.5, 26,1, 26.8, 23.2, 24.2, 28.4, 25.0, 27.3, and 25.7.

a. The manufacturer wants to be certain that the mean battery life exceeds 25 h. What conclusions canbe drawn from these data (use 0.05)?

b. Construct a 90% two-sided confidence interval on mean life in the accelerated test.

c. Construct a normal probability plot of the battery life data. What conclusions can you drawn?