When you use the data= argument in glm(), the function looks in the data.frame
for a variable first. You have created two versions of stype, one in the
data.frame and one outside it. So your first glm() selects all the cases
apistrat since apistrat$stype always equals apistrat$stype. You can see this
with
(b <- glm(api00~ell+meals+mobility, data = apistrat,
subset = stype == "E"))
gives the same results as
(a <- glm(api00~ell+meals+mobility, data = apistrat,
subset = apistrat$stype == "E"))
If you want to use a variable outside the data frame, give it another name,
e.g.:
styp <- "E"
(a <- glm(api00~ell+meals+mobility, data = apistrat,
subset = stype == styp))
-------------------------------------
David L Carlson
Department of Anthropology
Texas A&M University
College Station, TX 77840-4352
-----Original Message-----
From: R-help [mailto:[email protected]] On Behalf Of Ganz, Carl
Sent: Tuesday, March 21, 2017 10:50 AM
To: [email protected]
Subject: [R] Issue with subset in glm
Hello,
I am experiencing odd behavior with the subset parameter for glm. It appears
that the parameter uses non-standard evaluation, but only in some cases. Below
is a reproducible example.
library(survey) # for example dataset
data(api)
stype <- "E"
(a <- glm(api00~ell+meals+mobility, data = apistrat,
subset = apistrat$stype == stype))
(b <- glm(api00~ell+meals+mobility, data = apistrat,
subset = apistrat$stype == "E"))
# should be equal since stype = "E" but they aren't
coef(a)==coef(b)
# for some reason works as expected here
i = 4
(c <- glm(mpg ~ wt, data = mtcars, subset = mtcars$cyl==i))
(d <- glm(mpg ~ wt, data = mtcars, subset = mtcars$cyl==4))
coef(c)==coef(d)
I can't really explain what is happening so I would appreciate help.
Kind Regards,
Carl Ganz
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______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
