NAG CL Interface
g03ecc (cluster_​hier)

1 Purpose

g03ecc performs hierarchical cluster analysis.

2 Specification

#include <nag.h>
void  g03ecc (Nag_ClusterMethod method, Integer n, double d[], Integer ilc[], Integer iuc[], double cd[], Integer iord[], double dord[], NagError *fail)
The function may be called by the names: g03ecc, nag_mv_cluster_hier or nag_mv_hierar_cluster_analysis.

3 Description

Given a distance or dissimilarity matrix for n objects (see g03eac), cluster analysis aims to group the n objects into a number of more or less homogeneous groups or clusters. With agglomerative clustering methods, a hierarchical tree is produced by starting with n clusters, each with a single object and then at each of n-1 stages, merging two clusters to form a larger cluster, until all objects are in a single cluster. This process may be represented by a dendrogram (see g03ehc).
At each stage, the clusters that are nearest are merged, methods differ as to how the distance between the new cluster and other clusters are computed. For three clusters i , j and k let n i , n j and n k be the number of objects in each cluster and let d ij , d ik and d jk be the distances between the clusters. Let clusters j and k be merged to give cluster jk , then the distance from cluster i to cluster jk , d i . j k can be computed in the following ways:
  1. 1.Single link or nearest neighbour: d i . j k = min d ij , d ik .
  2. 2.Complete link or furthest neighbour: d i . j k = max d ij , d ik .
  3. 3.Group average: d i . j k = n j n j + n k d ij + n k n j + n k d ik .
  4. 4.Centroid: d i . j k = n j n j + n k d ij + n k n j + n k d ik - n j n k n j + n k 2 d jk .
  5. 5.Median: d i . j k = 1 2 d ij + 1 2 d ik - 1 4 d jk .
  6. 6.Minimum variance: d i . j k = n i + n j d ij + n i + n k d ik - n i d jk / n i + n j + n k .
For further details see Everitt (1974) or Krzanowski (1990).
If the clusters are numbered 1 , 2 , , n then, for convenience, if clusters j and k , j<k , merge then the new cluster will be referred to as cluster j . Information on the clustering history is given by the values of j , k and d jk for each of the n-1 clustering steps. In order to produce a dendrogram, the ordering of the objects such that the clusters that merge are adjacent is required. This ordering is computed so that the first element is 1. The associated distances with this ordering are also computed.

4 References

Everitt B S (1974) Cluster Analysis Heinemann
Krzanowski W J (1990) Principles of Multivariate Analysis Oxford University Press

5 Arguments

1: method Nag_ClusterMethod Input
On entry: indicates which clustering.
Single link.
Complete link.
Group average.
Minimum variance.
Constraint: method=Nag_SingleLink, Nag_CompleteLink, Nag_GroupAverage, Nag_Centroid, Nag_Median or Nag_MinVariance.
2: n Integer Input
On entry: the number of objects, n .
Constraint: n2 .
3: d[n×n-1/2] double Input/Output
On entry: the strictly lower triangle of the distance matrix. D must be stored packed by rows, i.e., d[ i-1 i-2 / 2 + j - 1 ] , i>j must contain d ij .
On exit: is overwritten.
Constraint: d[i-1] 0.0 , for i=1,2,, n n-1 / 2 .
4: ilc[n-1] Integer Output
On exit: ilc[l-1] contains the number, j , of the cluster merged with cluster k (see iuc), j<k , at step l , for l=1,2,,n - 1.
5: iuc[n-1] Integer Output
On exit: iuc[l-1] contains the number, k , of the cluster merged with cluster j , j<k , at step l, for l=1,2,,n-1.
6: cd[n-1] double Output
On exit: cd[l-1] contains the distance d jk , between clusters j and k , j<k , merged at step l , for l=1,2,,n - 1.
7: iord[n] Integer Output
On exit: the objects in dendrogram order.
8: dord[n] double Output
On exit: the clustering distances corresponding to the order in iord. dord[l-1] contains the distance at which cluster iord[l-1] and iord[l] merge, for l=1,2,,n - 1. dord[n-1] contains the maximum distance.
9: fail NagError * Input/Output
The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).

6 Error Indicators and Warnings

Dynamic memory allocation failed.
On entry, argument method had an illegal value.
A true dendrogram cannot be formed because the distances at which clusters have merged are not increasing for all steps, i.e., cd[i-1] < cd[i-2] for some i = 2 , 3 , , n - 1 . This can occur for the method=Nag_Centroid and method=Nag_Median methods.
On entry, n=value.
Constraint: n2.
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
On entry, d[value] = value.
Constraint: d[i-1] 0.0 , for i=1,2,,n × n-1 / 2.

7 Accuracy

For methods other than method=Nag_SingleLink or Nag_CompleteLink, slight rounding errors may occur in the calculations of the updated distances. These would not normally significantly affect the results, however there may be an effect if distances are (almost) equal.
If at a stage, two distances d ij and d kl , i<k or i=k and j<l , are equal then clusters k and l will be merged rather than clusters i and j . For single link clustering this choice will only affect the order of the objects in the dendrogram. However, for other methods the choice of kl rather than ij may affect the shape of the dendrogram. If either of the distances d ij or d kl are affected by rounding errors then their equality, and hence the dendrogram, may be affected.

8 Parallelism and Performance

g03ecc is not threaded in any implementation.

9 Further Comments

The dendrogram may be formed using g03ehc. Groupings based on the clusters formed at a given distance can be computed using g03ejc.

10 Example

Data consisting of three variables on five objects are read in. Euclidean squared distances based on two variables are computed using g03eac, the objects are clustered using g03ecc and the dendrogram computed using g03ehc. The dendrogram is then printed.

10.1 Program Text

Program Text (g03ecce.c)

10.2 Program Data

Program Data (g03ecce.d)

10.3 Program Results

Program Results (g03ecce.r)