Statistical power, accuracy, reproducibility and robustness of a graph clusterability test

Not all graphs are clusterable. Not all graphs have a clustered structure and can be meaningfully summarized through vertex clustering. Clusterable graphs are characterized by pockets of densely connected vertices that are only sparsely connected to the remaining graph. In this article, we re-introduce a very simple and intuitive, yet highly informative, statistical hypothesis test for graph clusterability that is based on vertex and neighborhood samples. The goal of this test is to determine if a graph meets the necessary structural conditions to be summarized meaningfully through vertex clusters. Our test is based on the hypothesis that a clusterable graph will display, on average, a local neighborhood induced subgraph density that is greater than the graph’s overall density. The test is also applied to graph comparisons, to test whether one graph has a stronger clustered structure than another. Significance is assessed using the t -statistic. Since it is based on sampling, we provide a focused examination of our test’s sensitivity to sample size. The main contribution of this article is a detailed examination of our test’s accuracy, sensitivity to sample size, conclusion reproducibility and robustness. Our empirical results remain consistent with our earlier conclusions and demonstrate the almost perfect accuracy of our test, even with very small samples of the graph. They also reveal that our test remains robust even under severe departures from the null hypothesis.