A large furniture store stations salespeople near its entrance to greet customers and offer assistance in shopping. The salespeople, who work on a commission basis, tell the customers their name and hand them a business card. A psychologist thinks that the salespersons' intrusiveness might cause customers to buy less furniture rather than more furniture. She convinces the store's management to let her study the issue. Customers are randomly selected to either receive or not receive a salesperson's offer of assistance immediately on entering the store. The amounts of customers' purchases are then logged as they leave the store. Here are the data:

a. What are the independent and dependent variables in this study?
b. State the null hypothesis and the directional (one-tailed) research hypothesis.
c. Calculate t and compare it with the tabled critical t at the .01 and .05 α levels. Can you reject the null hypothesis?

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Amount of Purchase, in U.S. Dollars Immediate Assistance No Assistance Unless Requested 761 2,592 1,037 84 672 2,274 362 855 1,273