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	Service Life (hours)
1	495	500	505	500
2	525	515	505	515
3	470	480	460	470

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

Multiple Choice

Sample 1

Sample 2

Sample 3

Both samples 2 and 3.