In a substudy, participants will be subdivided into two equal groups according to the severity of previous allergy symptoms (n = 30 each), and the effectiveness of the eye drop (vs. placebo) will be compared between the two groups. If 60 participants are enrolled in the main study (in the two groups combined), then how much power will the substudy have if there is a true mean difference in effectiveness of 0.25 [i.e., (mean change score active eye €“ mean change score placebo eye, subgroup 1) €“ (mean change score active eye €“ mean change score placebo eye, subgroup 2) = 0.25] between the two groups and a two-sided test is used with a significance level of .05?

Ophthalmology

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.

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Change score in active eye - change score in placebo eye Active Placebo eye eye Mean t sd Mean + sd Mean t sd Change in -0.61 + 0.70 -0.04 + 0.68 -0.57 + 0.86 average redness score (visit 2 – visit 1 score)