A new framework for clustering
The difficulty of clustering and the variety of clustering methods suggest the need for a theoretical study of clustering. Using the idea of a standard statistical framework, we propose a new framework for clustering. For a well-defined clustering goal we assume that the data to be clustered come from an underlying distribution and we aim to find a high-density cluster tree. We regard this tree as a parameter of interest for the underlying distribution. However, it is not obvious how to determine a connected subset in a discrete distribution whose support is located in a Euclidean space. Building a cluster tree for such a distribution is an open problem and presents interesting conceptual and computational challenges. We solve this problem using graph-based approaches and further parameterize clustering using the high-density cluster tree and its extension. Motivated by the connection between clustering outcomes and graphs, we propose a graph family framework. This framework plays an important role in our clustering framework. A direct application of the graph family framework is a new cluster-tree distance measure. This distance measure can be written as an inner product or kernel. It makes our clustering framework able to perform statistical assessment of clustering via simulation. Other applications such as a method for integrating partitions into a cluster tree and methods for cluster tree averaging and bagging are also derived from the graph family framework.