Efficient hierarchical clustering of large high dimensional datasets.

Hierarchical clustering is extensively used to organize high dimensional objects such as documents and images into a structure which can then be used in a multitude of ways. However, existing algorithms are limited in their application since the time complexity of agglomerative style algorithms can be as much as O(n2log n) where n is the number of objects. Furthermore the computation of similarity between such objects is itself time consuming given they are high dimension and even optimized built in functions found in MATLAB take the best part of a day to handle collections of just 10,000 objects on typical machines. In this paper we explore using angular hashing to hash objects with similar angular distance to the same hash bucket. This allows us to create hierarchies of objects within each hash bucket and to hierarchically cluster the hash buckets themselves. With our formal guarantees on the similarity of objects in the same bucket this leads to an elegant agglomerative algorithm with strong performance bounds. Our experimental results show that not only is our approach thousands of times faster than regular agglomerative algorithms but surprisingly the accuracy of our results is typically as good and can sometimes be substantially better.