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Community Analysis Package

15.4

Average linkage

Also known as group average sorting (Lance & Williams, 1966). At each step, the

clusters are compared in terms of the average similarity of their members and the

two clusters that are most similar are fused.

15.5

McQuitty's

If Dkl

 = any distance or dissimilarity measure between clusters 

Ck

 and 

Cl

The distance between two clusters can be defined either directly or

combinatorially (Lance and Williams 1967), that is, by an equation for updating a

distance matrix when two clusters are joined. In all of the following combinatorial

formulas, it is assumed that clusters 

Ck

 and 

Cl

 are merged to form 

Cm

, and the

formula gives the distance between the new cluster 

Cm

 and any other cluster 

Cj

.

The combinatorial formula is

Djm

 = [(

Djk

 + 

Djl

)/2]

The method was independently developed by Sokal and Michener (1958) and

McQuitty (1966).

15.6

Gower's.

At each step, the centroid of the clusters are compared and the two clusters that

are most similar are fused. The position of the centroid is calculated using the

formula of Gower(1967).

15.7

Centroid

At each iteration, the clusters are compared in terms of the similarity of their most

similar samples (columns) and the two clusters that hold the most similar samples

are fused. The average of the attributes of the fused group is calculated and the

similarity between average properties are used in subsequent iterations.

Once a measure has been selected the cluster analysis will immediately be run.

Output from the analysis is presented under a number of tabbed components that

can each be viewed by clicking on the tab.

Note that, depending on the distance measure used, the dendrogram may appear

in a recursive form i.e. where parent nodes are plotted further along the similarity

gradient than their child nodes. This is caused when the combination of samples

causes a change in observed species similarity. To present the dendrogram in a

more usual format, check the  Space equally  box.

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