Scheid et al. (A-31) questioned 387 women receiving free bone mineral density screening. The questions focused on past smoking history. Subjects undergoing hormone replacement therapy

(HRT), and subjects not undergoing HRT, were asked if they had ever been a regular smoker. In the HRT group, 29.3 percent of 220 women stated that they were at some point in their life a regular smoker. In the non–HRT group, 17.3 percent of 106 women responded positively to being at some point in their life a regular smoker. (Sixty-one women chose not to answer the question.) Construct a 95 percent confidence interval for the difference in smoking percentages for the two populations of women represented by the subjects in the study. What assumptions about the data are necessary?