StATS: How do the various clustering algorithms work? (January 31, 2006).

I'm working with someone on some clustering models for his microarray experiment. He asked how the various clustering algorithms work.

The K-means algorithm selects K centroid locations, and then assigns points to cluster I if that data point is closer to the I centroid than any other centroid. The clusters are then examined and the centroid locations are moved in an attempt to improve the separation of clusters. This process is repeated until the centroid locations converge.

There is a nice graphical simulation of this algorithm at

which requires the Java Runtime Environment 1.3 or later. This is a free download, from Sun Microsystems.

Hierarchical clustering starts with a set of N clusters consisting of the N individual items. The algorithm then matches the two clusters that are closest to one another. Now you have a system with N-1 clusters. Repeat the process until the number of clusters becomes 1.

The same web page has a graphical simulation of hierarchical clustering at:

The concept of distance changes when you talk about the distance between two clusters rather than the distance between two items. Single linkage defines the distance between two clusters in an optimistic fashion. It looks at the best case, or the distance of the two closest items. Complete linkage defines the distance between two clusters in a pessimistic fashion, by looking at the distance of the two furthest items. Average linking looks at the distance between every pair and computes the average.

A good graphical illustration of these three methods is at:

As a practical matter, the three linkage methods usually produce similar results, but single linkage can sometimes produce long snakelike clusters.

These are the two simplest and most commonly used clustering algorithms. There are many other clustering approaches, which I will try to describe when I have time.

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