Code covered by the BSD License

# (simple) Tool for estimating the number of clusters

### Kaijun Wang (view profile)

10 Feb 2007 (Updated )

12 validity indices, illustrate estimation of the number of clusters

[AR,RI,MI,HI]=RandIndex(c1,c2)
```function [AR,RI,MI,HI]=RandIndex(c1,c2)
%RANDINDEX - calculates Rand Indices to compare two partitions
% ARI=RANDINDEX(c1,c2), where c1,c2 are vectors listing the
% class membership, returns the "Hubert & Arabie adjusted Rand index".
% [AR,RI,MI,HI]=RANDINDEX(c1,c2) returns the adjusted Rand index,
% the unadjusted Rand index, "Mirkin's" index and "Hubert's" index.
%
% See L. Hubert and P. Arabie (1985) "Comparing Partitions" Journal of
% Classification 2:193-218

%(C) David Corney (2000)   		D.Corney@cs.ucl.ac.uk

if nargin < 2 | min(size(c1)) > 1 | min(size(c2)) > 1
error('RandIndex: Requires two vector arguments')
return
end

C=Contingency(c1,c2);	%form contingency matrix

n=sum(sum(C));
nis=sum(sum(C,2).^2);		%sum of squares of sums of rows
njs=sum(sum(C,1).^2);		%sum of squares of sums of columns

t1=nchoosek(n,2);		%total number of pairs of entities
t2=sum(sum(C.^2));	%sum over rows & columnns of nij^2
t3=.5*(nis+njs);

nc=(n*(n^2+1)-(n+1)*nis-(n+1)*njs+2*(nis*njs)/n)/(2*(n-1));

A=t1+t2-t3;		%no. agreements
D=  -t2+t3;		%no. disagreements

if t1==nc
AR=0;			%avoid division by zero; if k=1, define Rand = 0
else
AR=(A-nc)/(t1-nc);		%adjusted Rand - Hubert & Arabie 1985
end

RI=A/t1;			%Rand 1971		%Probability of agreement
MI=D/t1;			%Mirkin 1970	%p(disagreement)
HI=(A-D)/t1;	%Hubert 1977	%p(agree)-p(disagree)

function Cont=Contingency(Mem1,Mem2)

if nargin < 2 | min(size(Mem1)) > 1 | min(size(Mem2)) > 1
error('Contingency: Requires two vector arguments')
return
end

Cont=zeros(max(Mem1),max(Mem2));

for i = 1:length(Mem1);
Cont(Mem1(i),Mem2(i))=Cont(Mem1(i),Mem2(i))+1;
end
```