Spectral clustering in telephone call graphs

We evaluate various heuristics for hierarchical spectral clustering in large telephone call graphs. Spectral clustering without additional heuristics often produces very uneven cluster sizes or low quality clusters that may consist of several disconnected components, a fact that appears to be common for several data sources but, to our knowledge, not described in the literature. Divide-and-Merge, a recently described postfiltering procedure may be used to eliminate bad quality branches in a binary tree hierarchy. We propose an alternate solution that enables k-way cuts in each step by immediately filtering unbalanced or low quality clusters before splitting them further. Our experiments are performed on graphs with various weight and normalization built based on call detail records. We investigate a period of eight months of more than two millions of Hungarian landline telephone users. We measure clustering quality both by cluster ratio as well as by the geographic homogeneity of the clusters obtained from telephone location data. Although divide-and-merge optimizes its clusters for cluster ratio, our method produces clusters of similar ratio much faster, furthermore we give geographically much more homogeneous clusters with the size distribution of our clusters resembling to that of the settlement structure.