R help - Jan 2013 - dummy encoding in metafor

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.

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)

### 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?

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,


kind regards, 
Alma
	[[alternative HTML version deleted]]

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
info at aghmed.fsnet.co.uk
http://www.aghmed.fsnet.co.uk/home.html