Question: In a study done in Maryland, investigators surveyed individuals by telephone about how often they get tension headaches (Schwartz et al., 1998). One response variable that was measured was whether or not the respondent had experienced an episodic tension-type headache (ETTH) in the prior year. A headache pattern was called "episodic" if the headaches occurred less often than 15 times a month; otherwise, the headaches were called "chronic." Of the 1600 women in the survey aged 18 to 29, a total of 653 said that they had experienced ETTHs in the last year. Of the 2122 women in the 30- to 39-year-old age group, the number having experienced episodic headaches was 995.
a. Calculate the proportion in each group who experienced an episodic headache in the prior year, and compute the difference in these two proportions.
b. Compute a 95% confidence interval for the difference between the proportions for these age groups in the population. Write a sentence that interprets this confidence interval.