[R] Issue with subset in glm

[David L Carlson]

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:r-help-bounces at r-project.org] On Behalf Of Ganz, Carl
Sent: Tuesday, March 21, 2017 10:50 AM
To: r-help at r-project.org
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

______________________________________________
R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.