10.9. An Associated Press story (February 15, 2002)

quoted a study at the University of California at San Diego that reported, based on a nationwide survey, that those who averaged at least 8 hours sleep a night were 12% more likely to die within six years than those who averaged 6.5 to 7.5 hours of sleep a night.

(a) Explain how the subject's age could be positively associated both with time spent sleeping and with an increased death rate and hence could explain the association between sleeping and the death rate.

(b) If the association disappears when we control for subject's age, do you think age is more likely to be a common cause or an intervening variable?