Planning a study to compare ad placement
Refer to Exercise 7.80 (page 470), where we compared trustworthy ratings for ads from two different publications. Suppose that you are planning a similar study using two different publications that are not expected to show the differences seen when comparing the Wall Street Journal with the National Enquirer. You would like to detect a difference of 1.5 points using a two-sided signi?cance test with a 5% level of signi?cance. Based on Exercise 7.80, it is reasonable to use 1.6 as the value of the standard deviation for planning purposes.
(a) What is the power if you use sample sizes similar to those used in the previous study, for example, 65 for each publication?
(b) Repeat the calculations for 100 in each group.
(c) What sample size would you recommend for the new study?