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Suppose that Walmart has 1,000 stores. It has a 20% coupon for cereal, and it hypothesizes that the coupon will increase the sales of cereal by 3%.

Walmart wants to put the coupon in $n$ stores on 2023-06-01. Suppose that it has data from 2021-06-01 onward, and that it can install the coupon in those $n$ stores for as long as necessary.

At a significance level of 5%, I want to use difference-in-difference to estimate the change in sales in the $n$ test stores after 2021-06-01. What should $n$ be?

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