The National Health Interview Survey conducted of 27,603 adults by the U.S. National Center for Health Statistics in 2009 indicated that 20.6% of adults were current smokers. A similar study conducted in 1991 of 42,000 adults indicated that 25.6% were current smokers.
a. Find and interpret a point estimate of the difference between the proportion of current smokers in 1991 and the proportion of current smokers in 2009.
b. A 99% confidence interval for the true difference is (0.042, 0.058). Interpret.
c. What assumptions must you make for the interval in part b to be valid?

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