Message-ID: <8196661B-256A-4E87-8296-D245E717DDE8@uni-bremen.de>
Date: 2009-11-29T15:41:36Z
From: Karsten Wolf
Subject: How to z-standardize for subgroups?
Hi folks,
I have a dataframe df.vars with the follwing structure:
var1 var2 var3 group
Group is a factor.
Now I want to standardize the vars 1-3 (actually - there are many
more) by class, so I define
z.mean.sd <- function(data){
return.values <- (data - mean(data)) / (sd(data))
return(return.values)
}
now I can call for each var
z.var1 <- by(df.vars$var1, group, z.mean.sd)
which gives me the standardised data for each subgroup in a list with
the subgroups
z.var1 <- unlist(z.var1)
then gives me the z-standardised data for var1 in one vector. Great!
Now I would like to do this for the whole dataframe, but probably I am
not thinking vectorwise enough.
z.df.vars <- by(df.vars, group, z.mean.sd)
does not work. I banged my head on other solutions trying out sapply
and tapply, but did not succeed. Do I need to loop and put everything
together by hand? But I want to keep the columnnames in the vector?
-karsten
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Karsten D. Wolf
Didactical Design of Interactive
Learning Environments
Universit?t Bremen - Fachbereich 12
web: http://www.ifeb.uni-bremen.de/wolf/