R: Agglomerative / Divisive Coefficient for 'hclust' Objects

coef.hclust {cluster} | R Documentation

## Agglomerative / Divisive Coefficient for 'hclust' Objects

### Description

Computes the “agglomerative coefficient” (aka “divisive coefficient” for `diana`), measuring the clustering structure of the dataset.

For each observation i, denote by m(i) its dissimilarity to the first cluster it is merged with, divided by the dissimilarity of the merger in the final step of the algorithm. The agglomerative coefficient is the average of all 1 - m(i). It can also be seen as the average width (or the percentage filled) of the banner plot.

`coefHier()` directly interfaces to the underlying C code, and “proves” that only `object$heights` is needed to compute the coefficient.

Because it grows with the number of observations, this measure should not be used to compare datasets of very different sizes.

### Usage

    coefHier(object)
    coef.hclust(object, ...)
    ## S3 method for class 'hclust'
    coef(object, ...)
    ## S3 method for class 'twins'
    coef(object, ...)

### Arguments

`object` | an object of class `"hclust"` or `"twins"`, i.e., typically the result of `hclust(.)`,`agnes(.)`, or `diana(.)`. Since `coef.hclust` only uses `object$heights`, and `object$merge`, `object` can be any list-like object with appropriate `merge` and `heights` components. For `coefHier`, even only `object$heights` is needed.
`...` | currently unused potential further arguments

### Value

a number specifying the agglomerative (or divisive for `diana` objects) coefficient as defined by Kaufman and Rousseeuw, see `agnes.object $ ac` or `diana.object $ dc`.

### Examples

    data(agriculture)
    aa <- agnes(agriculture)
    coef(aa) # really just extracts aa$ac
    coef(as.hclust(aa))# recomputes
    coefHier(aa)       # ditto

[Package cluster version 2.0.9 Index]