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st: RE: ratios first or last?


From   Alexander Tsai <act2@cwru.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: RE: ratios first or last?
Date   Sat, 22 Nov 2003 23:15:46 -0500

Dana Goldman has some discussion of this in a recent publication:

Goldman DP, Smith JP. Methodological biases in estimating the burden of
out-of-pocket expenses. Health Serv Res. 2001 Feb;35(6):1357-64;
discussion 1365-70. Comment on: Health Serv Res. 1999 Apr;34(1 Pt
2):241-54.

Cheers,
Alex Tsai

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of David Airey
Sent: Saturday, November 22, 2003 8:27 PM
To: statalist@hsphsun2.harvard.edu
Subject: st: ratios first or last?


Recently I was surprised to find a difference between two methods of
calculating a ratio during an experiment. Each animal has two measures
taken repeatedly over time. The ratio is of the two measures. I could
take the ratio at each time point, and then average the ratios to get
my animal ratio. Alternatively, I could average each of the two
measures and then form a ratio of the two averages, again getting my
animal ratio. The second method consistently gets a higher ratio than
the first method. Why would this occur? The second method is standard
in my literature base.

-Dave

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