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st: A regression question.

From   "Puneet K Singhal" <>
Subject   st: A regression question.
Date   Fri, 19 Mar 2004 13:47:52 -0500

I have expenditures post-stroke at one month intervals and want to predict 
these expenditures by number of months post stroke controlling for other 
covariates.  For this purpose, I am using dummy variables for the number of 
months post stroke.  For example, if I have 12 months of follow-up I'm using 
11 dummies for each month up to 11 months and keeping 12th month as my ref 
category.  Now, after fitting this OLS I want to take the mean of predicted 
values for each month and compare the predicted values across months.  When I 
do that what I find is that the mean predicted values are the same as the 
mean observed values for each month.  I just can't conceptualize why they are 
the same.  When I stratify the predicted values by gender, I again get the 
same values as I would if I stratified the observed values by gender. 

Can someone help?

Puneet K Singhal, M.S.
Doctoral Candidate
Pharmaceutical Health Services Research Program
University of Maryland
Baltimore, MD-21201
Phone: 410-706-1805
Fax: 410-706-1488

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