[R] hcluster with linkage median

[Kennedy]

Hi,

I want to perform a hierarchical clustering using the median as linkage
metric. As I understand it the function hcluster in package amap have this
option but it does not produce the results that I expect.

In the example below M is a matrix of similarities that is transformed into
a matrix of dissimilarities D. 
> D
     [,1] [,2] [,3] [,4] [,5]
[1,]  1.0  0.9  0.2  0.2  0.1
[2,]  0.9  1.0  0.7  1.0  0.0
[3,]  0.2  0.7  1.0  0.8  0.8
[4,]  0.2  1.0  0.8  1.0  0.5
[5,]  0.1  0.0  0.8  0.5  1.0

Since [2,5]=0 the objects 2 and 5 should be grouped together in the first
step as is done by the agnes function but hcluster start by clustering
objects 3 and 4. Why is this?

Regards

Henrik

library(cluster)
library(amap)

# Create matrix M
M <- matrix(nr=5,nc=5)
M[,1] <- c(0,1,8,8,9) 
M[,2] <- c(1,0,3,0,10)
M[,3] <- c(8,3,0,2,2)
M[,4] <- c(8,0,2,0,5)
M[,5] <- c(9,10,2,5,0)

# Create matrix D
n <- dim(M)[1]
o <- matrix(1,n,n)
mn <- (1/max(M))*M
D <- o-mn

# Clustering using hcluster
ce <- hcluster(D,link="median")
plot(ce)

# Clustering using agnes
av <- agnes(D,diss=T,method="average")
pltree(av)
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