Chapter 8, Problem 104P

A study is being planned to assess whether a topical antiallergic eye drop is effective in preventing the signs and symptoms of allergic conjunctivitis. In a pilot study, at an initial visit, participants are given an allergen challenge; that is, they are subjected to a substance that provokes allergy signs (e.g., cat dander) and their redness score is noted 10 minutes after the allergen challenge (visit 1 score). At a follow-up visit, the same procedure is followed, except that participants are given an active eye drop in one eye and the placebo in the fellow eye 3 hours before the challenge; a visit 2 score is obtained 10 minutes after the challenge. The data collected are shown in Table 8.26.

Table 8.26 Effect of an eye drop in reducing ocular redness among participants subjected to an allergen challenge

^aThe redness score ranges from 0 to 4 in increments of 0.5, where 0 is no redness at all and 4 is severe redness.

Suppose 60 participants are enrolled in the main study. How much power would the study have to detect a 0.5-unit mean difference if a two-sided test is used with a significance level of .05?