10.61 Change coffee brand? A study was conducted to see if an advertisement campaign would increase market share for Sanka instant decaffeinated coffee (R. Grover and V.

Srinivasan, J. Marketing Research, vol. 24, 1987, pp. 139–

153). Subjects who use instant decaffeinated coffee were asked which brand they bought last. They were asked this before the campaign and after it. The results are shown in the table, with computer output based on it below.

a. Estimate the population proportion choosing Sanka for the (i) first purchase and (ii) second purchase. Find the difference and interpret it.

b. Explain how to interpret the 95% confidence interval.

c. The software output also shows a P-value. State the hypotheses that this P-value refers to in the context of this exercise and give the conclusion of the test when using a significance level of 0.05.

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Two Purchases of Coffee First Purchase Sanka Second Purchase Other Brand Sanka 155 49 Other brand 76 261 McNemar's Test Estimated Difference -0.0499 (-0.0920, -0.0078) P 0.020 Difference = p (First Purchase) p (Second 95% CI Purchase Sanka)