what is the actual value of our power?
Suppose that we are planning a study to compare three treatments for depression. Group 1 subjects receive weekly therapy sessions using client-centered therapy. Group 2 subjects also receive client centered therapy but are seen only every two weeks. Group 3 subjects serve as a waiting list control group. Posttest assessment occurs 3 months into the study. The dependent measure is the Center for Epidemiology Studies’ Depression Scale (CES-D). a. Our best guess as to the likely magnitude of group differences is reflected in the following population means: ^ = 15, /z2 = 18, and ^ = 24. We expect the population standard deviation (within-groups) to be around 10. Naturally, we set a at .05. What is the total number of subjects we should include in our study, assuming equal n per group in order to have a power of .8? b. Suppose that our estimate of the population standard deviation in part a is too small. Specifically, assume that the true value is 14 instead of 10. Because we planned our study using the value of 10, the number of subjects we use is still the number you found in part a. If we use this many subjects, but in fact 14 is the true standard deviation, what is the actual value of our power?
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