[R] selecting rows for inclusion in lm

[John Sorkin]

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 at grecc.umaryland.edu

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