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:
What is the standard deviation of the sampling distribution of sample means for whenever service life is in control?
Select one:
A. 10 hours
B. 6.67 hours
C. 11.55 hours
D. 5 hours
E. 20 hours