Given:
71% of all shoppers use coupons.
18 times out of 20 these results were considered accurate to within 2.5%.
71% represents the percentage of the whole population which is all the shoppers of grocery store.
18 times out of 20 refers to the confidence interval of the results.
18/20 * 100% = 90%
2.5% is the range of accuracy in the 90% confidence interval.
The margin of error is 2.5%.
