[R] statistical test for comparison of two classifications (nominal)

[Matt Shotwell]

There are several statistics used to compare nominal classifications, or
_partitions_ of a data set. A partition isn't quite the same in this
context because partitioned data are not restricted to a fixed number of
classes. However, the statistics used to compare partitions should also
work for these 'restricted' partitions. See the Rand index, Fowlkes and
Mallows index, Wallace indices, and the Jaccard index. The profdpm
package implements a function (?profdpm::pci) that computes these
indices for two factors representing partitions of the same data.

The difficult part is drawing statistical inference about these indices.
It's difficult to formulate a null hypothesis, and even more difficult
to determine a null distribution for a partition comparison index. A
bootstrap test might work, but you will probably have to implement this
yourself.

-Matt

On Wed, 2010-11-17 at 08:33 -0500, Martin Tomko wrote:
> Dear all,
> I am having a hard time to figure out a suitable test for the match 
> between two nominal classifications of the same set of data.
> I have used hierarchical clustering with multiple methods (ward, 
> k-means,...) to classify my dat into a set number of classesa, and I 
> would like to compare the resulting automated classification with the 
> actual - objective benchmark one.
> So in principle I have a data frame with n columns of nominal 
> classifications, and I want to do a mutual comparison and test for 
> significance in difference in classification between pairs of columns.
> 
> I just need to identify a suitable test, but I fail. I am currently 
> exploring the possibility of using Cohen's Kappa, but I am open to other 
> suggestions. Especially the fact that kappa seems to be moslty used on 
> failible, human annotators seems to bring in limitations taht do not 
> apply to my automatic classification.
> Any help will be appreciated, especially if also followed by a pointer 
> to an R package that implements it.
> 
> Thanks
> Martin
> 
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-- 
Matthew S. Shotwell
Graduate Student 
Division of Biostatistics and Epidemiology
Medical University of South Carolina