At 17:14 19/01/2013, Alma Wilflinger wrote:
Hi,
I am quite new to R and in need of some advice. I am trying to
conduct a meta regression over a some studies with about 7 mod
variables which I have to dummy encode.
Alma, although you can generate your own dummy variables by hand you
do not have to as R will do it for you. See below for more comments.
I have found the following piece of code in the manual for the
metafor library:
### manual dummy coding of the allocation factor
alloc.random <- ifelse(dat$alloc == "random", 1, 0)
alloc.alternate <- ifelse(dat$alloc == "alternate", 1, 0)
alloc.systematic <- ifelse(dat$alloc == "systematic", 1, 0)
If you look a bit further down the manual page you will see
### using a model formula to specify the same model
rma(yi, vi, mods=~factor(alloc)+year+ablat, data=dat, method="REML",
btt=c(2,3))
which is much easier.
### test the allocation factor (in the presence of the other moderators)
### note: "alternate" is the reference level of the allocation factor
### note: the intercept is the first coefficient, so btt=c(2,3)
rma(yi, vi, mods=cbind(alloc.random, alloc.systematic, year, ablat),
data=dat, method="REML", btt=c(2,3))
What I do not understand is the following:
How does R know which columns in my data.frame are related to the
dummy encoded variables?
If you code them yourself R does not know. You know.
It is clear that in the call of cbind I just do not use the
reference variable as a parameter but I do not get it how R knows
that alloc.random and alloc.systematic refer to the column alloc in
the data frame.
Thank you very much in advance for your help,
You say you have seven moderator variables. Unless you have a shed
load of studies you will not be able to look at them simultaneously.
Apologies if you already knew that.
kind regards,
Alma
[[alternative HTML version deleted]]
Michael Dewey
i...@aghmed.fsnet.co.uk
http://www.aghmed.fsnet.co.uk/home.html
______________________________________________
R-help@r-project.org mailing list
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.
