[R] Getting group-wise standard scores of a vector

[Erik Iverson]

Hello -

First, I doubt you really want to cbind() those two vectors within the 
data.frame() function call.

test.data <- data.frame(x, group)  is probably what you want.  That may 
be the source of your trouble.

If you really want a vector returned, the following should work given 
your test.data is constructed without the cbind():

unlist(by(test.data$x, test.data$group, function(x) (x - mean(x)) / 
sd(x)), use.names = FALSE)

Is that what you're after?

Erik


Matthew Dubins wrote:
> Hi,
> 
> I want to be able to create a vector of z-scores from a vector of 
> continuous data, conditional on a group membership vector.
> 
> Say you have 20 numbers distributed normally with a mean of 50 and an sd 
> of 10:
> 
> x <- rnorm(20, 50, 10)
> 
> 
> Then you have a vector that delineates 2 groups within x:
> 
> group <- sort(rep(c("A", "B"), 10))
> 
> test.data <- data.frame(cbind(x, group))
> 
> I know that if you break up the x vector into 2 different vectors then 
> it becomes easy to calculate the z scores for each vector, then you 
> stack them and append them to the original
> data frame.  Is there anyway to apply this sort of calculation without 
> splitting the original vector up?  I tried a really complex ifelse 
> statement but it didn't seem to work.
> 
> Thanks in advance,
> Matthew Dubins
> 
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