Final answer:
The confidence level is 95% in this case.
Step-by-step explanation:
The confidence level represents the level of certainty that the results obtained from a sample are accurate and can be generalized to the entire population. In this case, the confidence level is not explicitly stated in the question, but it can be inferred from the information provided. It is mentioned that 18 times out of 20, the results were considered accurate to within 2.5%. This suggests that the confidence level is 95%, meaning that there is a 95% chance that the true proportion of shoppers using coupons lies within 2.5% of the observed proportion.