To determine the effect of advertising in the Yellow Pages, Bell Telephone took a sample of 40 retail stores that did not advertise in the Yellow Pages last year but did so this year. The annual sales (in thousands of dollars) for each store in both years were recorded.

a. Estimate with 90% confidence the improvement in sales between the two years.

b. Can we infer that advertising in the Yellow Pages improves sales?

c. Check to ensure that the required condition(s) of the techniques used in parts (a) and (b) is satisfied.

d. Would it be advantageous to perform this experiment with independent samples? Explain why or why not.