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 be a lurking variable responsible for the observed association between sleeping and the death rate.
b. Explain how the subject’s age could be a common cause of both variables.