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

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

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?

Multiple Choice

sample 1

all samples are in control

sample 2

both samples 2 and 3

sample 3