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st: replicating 2 X 2 data from a paper


From   "Ariel Linden, DrPH" <ariel.linden@gmail.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: replicating 2 X 2 data from a paper
Date   Tue, 2 Apr 2013 20:16:42 -0400

Hi Fellow Statalisters,

I am having a problem conceptualizing how to replicate data from a paper I
am reading  (Bourbeau et al. 2003).* They have two groups (treatment = 96,
control= 95) who had 152 and 158 baseline hospitalizations, respectively. 

They describe in the text: " the comparison of the proportion of hospital
admissions or emergency department and medical visits was based on the chi2
test. The Fisher exact test was used when the frequencies were small.
Percent difference effects of the intervention were calculated by dividing
the absolute difference between the intervention and usual care group
values."

The table (table 2) shows the difference as 3.9% and a p-value of 0.45.

So with this minimal information, I'd like to replicate the results, perhaps
using a 2 X 2 table format and test with chi2. 

My basic math (see below) does not seem to replicate the difference they
present. I get close using (treat-usual)/treat, but that is not consistent
with their text. Using a rate difference, I get something completely
different. In any case, I am not sure if there is a simple way to reproduce
these results consistent with how they claim to get there...

		usual	treat		(treat-usual)/treat
admits		152	158		0.038
N		95	96		
admits/N	1.6	1.65		0.046

* Bourbeau JB, et al. Reduction of hospital utilization in patients with
chronic obstructive pulmonary disease. Arch Intern Med 2003: 163:585-591.

Any help would be appreciated!

Ariel


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