Bisphenol A (BPA) is in the lining of most canned goods, and recent studies have shown a positive association between BPA exposure and behavior and health problems. How much does canned soup consumption increase urinary BPA concentration? That was the question addressed in a recent study in which consumption of canned soup over five days was associated with a more than 1000% increase in urinary BPA. In the study, 75 participants ate either canned soup or fresh soup for lunch for five days. On the fifth day, urinary BPA levels were measured. After a two-day break, the participants switched groups and repeated the process. The difference in BPA levels between the two treatments was measured for each participant. The study reports that a 95% confidence interval for the difference in means (canned minus fresh) is 19.6 to 25.5 μg/L.

(a) Is this a randomized comparative experiment or a matched pairs experiment? Why might this type of experiment have been used?
(b) What parameter are we estimating?
(c) Interpret the confidence interval in terms of BPA concentrations.
(d) If the study had included 500 participants instead of 75, would you expect the confidence interval to be wider or narrower?