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st: metan command & binary variables


From   <cthompson@dfpm.utah.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: metan command & binary variables
Date   Tue, 20 Apr 2004 17:22:19 -0600

Hello All --
I'm using Stata version 8.2 and I hope someone can clarify a 
question I have regarding the -metan- command and it's 
estimation of odds ratios versus risk ratios.  Suppose I have 
two rates that capture success on a diet where the first rate 
represents those who adhered to a certain protocol and the 
second rate represents those who dropped from the protocol 
after follow-up.  Each rate is captured by the number 
successful divided by the number in the respective group 
(adhere vs follow-up).  I created four variables:  num1, den1, 
num2, & den2 where num1 is the # of successful dieters that 
adhered & den1 is the # of adherers; a similar interpretation 
follows for num2 & den2.  When I run the following code

-metan num1 den1 num2 den2, random or label(namevar=Trial)-

I get an 'odds ratio' that works out to equal the ratio of the 
rate from the adherers over the rate for the droppers.  If I 
specify -rr- instead of -or- the results differ and I am at a 
loss as to why.  Consider the following values:
num1=4, den1=5, num2=4, den2=51    
where the calculation of (4/5)/(4/51) = 10.2, which is 
consistent with -metan..., or- but if I specify -metan..., rr- 
I get a 'risk ratio' of 6.111.  Am I not understanding the 
vernacular or can someone clue me into how the 6.111 was 
calculated?  

As always, your input is much appreciated!
thank you -- Clint Thompson    
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