[R] RE: z-scores for different factor levels

[Warnes, Gregory R]

Brian Ripley pointed out that there is already an R function scale() that
does the work of my standardize().

-Greg


-----Original Message-----
From: Warnes, Gregory R 
Sent: Friday, January 18, 2002 9:38 AM
To: 'Stuart Leask'
Cc: 'r-help at stat.math.ethz.ch'
Subject: RE: z-scores for different factor levels


Hi Stuart,

I often use this small function

	standardize <- function(x)  ( x - mean(x, na.rm=T) ) / sqrt(var(x,
na.rm=T))

to standardize variables. 

You should be able to use this to do what you want by splitting the data
frame into sections based on the factor level, using standardize() to create
a new variable in each section, then paste the data frame back together. 

Something like:

	# create test data
	test <- data.frame( f=sample(c("A","B","C"),100,replace=T), 
                           x1=rnorm(100,2,5), x2=rnorm(100) )
	# split by factor
	subsets <- split(test,test$f)

	# create the standardized version of x1
	subsets <- lapply(subsets, function(s) { s$x1s <- standardize(s$x1);
s} )

	# paste the results back together
	result <- data.frame()
	for(s in subsets) result <- rbind(result, s)

-Greg

-----Original Message-----
From: Stuart Leask [mailto:stuart.leask at nottingham.ac.uk]
Sent: Friday, January 18, 2002 5:33 AM
To: r-help at stat.math.ethz.ch
Subject: [R]: z-scores for different factor levels


Hi there.
I am trying to generate z-scores for a variable according to it's factor
level, leaving this result in the original dataframe.

ie. standardised birth weight for gestational age in weeks.
BWT is birthweight
GEST is gestational age in weeks (equivalent to the factor level)

I can generate the factor level mean & SD using tapply
tapply(BWT,GEST,mean) etc.
but this creates a new array of means & SDs.

- Can anyone suggest how I can slot these means & SDs by factor level
straight back into the original dataframe, so I can then subtract the mean &
divide by the SD to get a Z-score for each case?
- Is there a function already available that can generate z-scores by factor
levels?

Stuart

Dr Stuart Leask MA MRCPsych, Clinical Lecturer in Psychiatry
University of Nottingham Dept of Psychiatry, Duncan Macmillan House
Porchester Road, Nottingham. NG3 6AA. UK
http://www.nottingham.ac.uk/psychiatry/staff/sjl.html

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