Will they charge more? In Exercise 21.32, you carried out the calculations for a confidence interval based on a bank’s experiment in changing the rules for its credit cards. You ought to ask some questions about this study.

(a) The distribution of the amount charged is skewed to the right, but outliers are prevented by the credit limit that the bank enforces on each card. Why can What Is a Confidence Interval?

we use a confidence interval based on a Normal sampling distribution for the sample mean x?

(b) The bank’s experiment was not comparative. The increase in amount charged over last year may be explained by lurking variables rather than by the rule change. What are some plausible reasons why charges might go up? Outline the design of a comparative randomized experiment to answer the bank’s question.