A design engineer wants to construct a sample mean chart for controlling the service life of a halogen headlamp his company produces. He knows from numerous previous samples that this service life is normally distributed with a mean of 500 hours and a standard deviation of 20 hours. On three recent production batches, he tested service life on random samples of four headlamps, with these results:
1 495 500 500 505
2 505 515 515 525
3 460 470 470 480
If he uses upper and lower control limits of 520 and 480 hours, what is his risk α of concluding service life is out of control when it is actually under control (Type I error)?
If he uses upper and lower control limits of 520 and 480 hours, on what sample(s) (if any) does service life appear to be out of control?