Do people behave differently on Friday the 13th? Researchers collected data on the number of shoppers at a random sample of 45 grocery stores on Friday the 6th and Friday the 13th in the same month. Then they calculated the difference (subtracting in the order 6th minus 13th) in the number of shoppers at each store on these 2 days. The mean difference is  and the standard deviation of the differences is 178.0.

a. If the result of this study is statistically significant, can you conclude that the difference in shopping behavior is due to the effect of Friday the 13th on people’s behavior? Why or why not?

b. Do these data provide convincing evidence at the level that the number of shoppers at grocery stores on these 2 days differs, on average?

c. Based on your conclusion in part (a), which type of error—a Type I error or a Type II error—could you have made? Explain your answer.

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305 0.200 = 20.0%46.5 1526