A design engineer wants to construct a sample mean chart for controlling the service life of one of the types of light bulbs his company produces. The engineer 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, the engineer tested service life on random samples of four bulbs, with these results:

Sample

2

3

495

525

470

Service Life (hours)

500

505

515

505

480

460

500

515

470

upper and lower control limits of 520 and 480 hours are used, on what samples) (if any) does service life appear to be out of control?

Multiple Choice

Sample 1

Sample 2