[R] How to z-standardize for subgroups?

[Chuck Cleland]

On 11/29/2009 4:23 PM, John Kane wrote:
> http://finzi.psych.upenn.edu/R/library/QuantPsyc/html/Make.Z.html
> 
> Make.Z in the QuantPsych package may already do it.

  For a single variable, you could use ave() and scale() together like this:

with(iris, ave(Sepal.Width, Species, FUN = scale))

  To scale more than one variable in a concise call, consider something
along these lines:

apply(iris[,1:4], 2, function(x){ave(x, iris$Species, FUN = scale)})

hope this helps,

Chuck Cleland

> --- On Sun, 11/29/09, Karsten Wolf <wolf at uni-bremen.de> wrote:
> 
>> 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
>> Universität Bremen - Fachbereich 12
>> web: http://www.ifeb.uni-bremen.de/wolf/
>>
>> ______________________________________________
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>>
> 
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-- 
Chuck Cleland, Ph.D.
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