To judge the effectiveness of an advertising campaign for a certain brand of potato chip, a random sample of 500 supermarket receipts was taken during the week before the ad campaign began, and 65 of them showed a purchase of the potato chips. Another random sam- ple of 500 receipts was taken during the week after the campaign ended, and 92 of them showed a potato chip purchase. Find a 99% confidence interval for the difference in the proportions of customers purchasing potato chips before and after the ad campaign.