[R] selecting rows for inclusion in lm

From: John Sorkin <jsorkin_at_grecc.umaryland.edu>
Date: Thu 18 Jan 2007 - 07:32:59 GMT


I am having trouble selecting rows of a dataframe that will be included in a regression. I am trying to select those rows for which the variable Meno equals PRE. I have used the code below:

difffitPre<-lm(data[,"diff"]~data[,"Age"]+data[,"Race"],data=data[data[,"Meno"]=="PRE",]) summary(difffitPre)

The output from the summary indicates that more than 76 rows are included in the regression:

Residual standard error: 2.828 on 76 degrees of freedom

where in fact only 22 rows should be included as can be seen from the following:

print(data[length(data[,"Meno"]=="PRE","Meno"])) [1] 22

I would appreciate any help in modifying the data= parameter of the lm so that I include only those subjects for which Meno=PRE.

R 2.3.1
Windows XP

Thanks,
John

John Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
Baltimore VA Medical Center GRECC,
University of Maryland School of Medicine Claude D. Pepper OAIC, University of Maryland Clinical Nutrition Research Unit, and Baltimore VA Center Stroke of Excellence

University of Maryland School of Medicine Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524

(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)
jsorkin@grecc.umaryland.edu

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