Finding the causes of the major mental illnesses, schizophrenia and bipolar disorder, is a major activity in medical research. Tkachev et al. (2003) compared expression levels of several genes involved in the production and activity of myelin, a tissue important in nerve function, in the brains of 15 persons with schizophrenia, 15 with bipolar illness, and 15 control individuals. The results presented in the table at the bottom of this page summarize the expression levels of the proteolipid protein 1 gene (PLP1) in the 45 brains.

a. The main objective of the study was to compare PLP1 gene expression in persons having schizophrenia with that of control individuals. Using a planned comparison approach, compute a 95% confidence interval for the difference between the means of these two groups.

b. Is a planned comparison appropriate in part (a)? Explain.

c. What are your assumptions in part (a)?