[R] tapply and weighted means

[Gavin Simpson]

On Thu, 2006-01-12 at 15:44 +0100, Florent Bresson wrote:
> I' m trying to compute weighted mean on different
> groups but it only returns NA. If I use the following
> data.frame truc:
> 
> x  y  w
> 1  1  1
> 1  2  2
> 1  3  1
> 1  4  2
> 0  2  1
> 0  3  2
> 0  4  1
> 0  5  1
> 
> where x is a factor, and then use the command :
> 
> tapply(truc$y,list(truc$x),wtd.mean, weights=truc$w)
> 
> I just get NA. What's the problem ? What can I do ?

Florent,

I guess you didn't read the help for tapply, which in the Value section
states:

     Note that optional arguments to 'FUN' supplied by the '...'
     argument are not divided into cells.  It is therefore
     inappropriate for 'FUN' to expect additional arguments with the
     same length as 'X'.

So tapply is not the right tool for this job. We can use by() instead (a
wrapper for tapply) as so:

dat <- matrix(scan(), byrow = TRUE, ncol = 3)
1  1  1
1  2  2
1  3  1
1  4  2
0  2  1
0  3  2
0  4  1
0  5  1

colnames(dat) <- c("x", "y", "w")
dat <- as.data.frame(dat)
dat
(res <- by(dat, dat$x, function(z) weighted.mean(z$y, z$w)))

but if you want to easily access the numbers you need to do a little
work, e.g.

as.vector(res)

Also, I don't see a function wtd.mean in standard R and weighted.mean()
doesn't have a weights argument, so I guess you are using a function
from another package and did not tell us.

HTH,

Gav
-- 
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Gavin Simpson                     [T] +44 (0)20 7679 5522
ENSIS Research Fellow             [F] +44 (0)20 7679 7565
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