Question: A 1986 clinical trial evaluated the effect of captopril on kidney function in insulin-dependent diabetic patients with nephropathy. Based on an analysis of systolic blood pressures of 16 randomized patients measured before and after one week’s treatment with captopril (n = 9) or placebo (n = 7), the authors concluded that “captopril represents a valuable new drug for treating hypertension in diabetics dependent on insulin with nephropathy”. This conclusion was based on paired t-tests computed on the data in each group. The authors reported that captopril (for nine patients) significantly reduced systolic blood pressure (p-value < 0.05) whereas “no significant changes in occurred in seven patients treated with placebo”.

Fortunately, the authors reported the original data in their research paper so readers could examine the statistical analysis that led the authors to their inference about the effect of captopril on systolic blood pressure. The data are provided in the attached Stata data file, captopril.dta and the file is described below.

. desc

Contains data from E:\captopril.dta

obs: 16 vars: 4 size: 160 (99.9% of memory free)

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storage display value

variable name type format label variable label

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id byte %8.0g patient id

baseline int %8.0g baseline SBP (mm Hg)

wk1 int %8.0g SBP after 1 week

cpril byte %8.0g captopril = 1 placebo = 0

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(a) Using this data file reproduce the results on systolic blood pressure given in Table IV of the research paper.

(b) What is wrong with their analysis, and their interpretation of the results?

(c) Perform a correct analysis. Use graphical and statistical methods to analyze the systolic blood pressure data.

(d) What conclusions can be drawn about the effect of captopril on systolic blood pressure? Write a summary of your statistical findings and conclusions.