[R]: z-scores for different factor levels

[Stuart Leask]

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

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._