How to z-standardize for subgroups?

From: Karsten Wolf <wolf at uni-bremen.de>
Subject: [R] How to z-standardize for subgroups?
To: r-help at r-project.org
Received: Sunday, November 29, 2009, 10:41 AM
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


---------------------------------------------------------------------------------------------
Karsten D. Wolf
Didactical Design of Interactive
Learning Environments
Universit?t Bremen - Fachbereich 12
web: http://www.ifeb.uni-bremen.de/wolf/