using weighted.mean with tapply()

I am trying to calculate the weighted mean for a of 10 deciles and I
get an error:
decile <- tapply(X=mat$trt1m, INDEX=mat$Rank, FUN=weighted.mean, w=mat$mcap)
Error in FUN(X[[1]], ...) : 'x' and 'w' must have the same length

All three of my inputs have the same length, as shown below, and the
weighted.mean calculation works by itself, just not in tapply()
length(mat$Rank)
[1] 1853
length(mat$mcap)
[1] 1853
length(mat$trt1m)
[1] 1853
mean(mat$trt1m)
[1] -0.04475397
weighted.mean(mat$trt1m, w=mat$mcap)
[1] -0.04819243
mat$mcap[is.na(mat$mcap)] <- min(mat$mcap, na.rm=TRUE)
I am probably making a simple error in how I pass the optional
parameter w.  Any help would be greatly appreciated.
I am trying to calculate the weighted mean for a of 10 deciles and I
get an error:

decile <- tapply(X=mat$trt1m, INDEX=mat$Rank, FUN=weighted.mean, w=mat$mcap)
Error in FUN(X[[1]], ...) : 'x' and 'w' must have the same length

All three of my inputs have the same length, as shown below, and the
weighted.mean calculation works by itself, just not in tapply()

Hi -- I asked pretty much this same question some years ago:

http://www.r-project.org/nocvs/mail/r-help/1999/2160.html

The answer turns out to be that you should pass the index to tapply
rather than the data. In your case this would, I think, translate to

decile <- tapply(seq(along=mat$trlm, mat$Rank,
function(i, x=mat$trlm[i], w=mat$mcap[i])
weighted.mean(x[i], w[i]))

hope this helps.

regards,

Markus

length(mat$Rank)
[1] 1853

length(mat$mcap)
[1] 1853

length(mat$trt1m)
[1] 1853

mean(mat$trt1m)
[1] -0.04475397
weighted.mean(mat$trt1m, w=mat$mcap)
[1] -0.04819243

mat$mcap[is.na(mat$mcap)] <- min(mat$mcap, na.rm=TRUE)

I am probably making a simple error in how I pass the optional
parameter w.  Any help would be greatly appreciated.

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Markus Jantti
Abo Akademi University
markus.jantti at iki.fi
http://www.iki.fi/~mjantti
I am trying to calculate the weighted mean for a of 10 deciles and I
get an error:
decile <- tapply(X=mat$trt1m, INDEX=mat$Rank, FUN=weighted.mean, w=mat$mcap)
Error in FUN(X[[1]], ...) : 'x' and 'w' must have the same length

All three of my inputs have the same length, as shown below, and the
weighted.mean calculation works by itself, just not in tapply()

length(mat$Rank)
[1] 1853
length(mat$mcap)
[1] 1853
length(mat$trt1m)
[1] 1853
mean(mat$trt1m)
[1] -0.04475397
weighted.mean(mat$trt1m, w=mat$mcap)
[1] -0.04819243
mat$mcap[is.na(mat$mcap)] <- min(mat$mcap, na.rm=TRUE)
I am probably making a simple error in how I pass the optional
parameter w.  Any help would be greatly appreciated.
When you use tapply, only the first argument of the function you supply
is split by the index variable.  There is no way for tapply to know if
further arguments should be split or not, so it doesn't split them.  
People are regularly caught out by this.  If you look carefully at the
documentation for split, there is a warning about it.

Here is a very dense solution (due to Peter Dalgaard):

by(mat, mat$Rank, with, weighted.mean(trt1m,  mcap))

Alternatively you can split up the data frame using split, write your
own custom wrapper function for doing the weighted mean, and use lapply
or sapply:

sapply(split(mat,mat$Rank), function(x) {weighted.mean(x$trt1m,x$mcap)})

Martyn

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