17. The computer storage medium of claim 16 , wherein identifying one or more candidate clusters of observations responsive to the generated query comprises: identifying one or more candidate clusters of observations responsive to the generated query using the cluster index.
17. The computer storage medium of claim 16 , wherein identifying one or more candidate clusters of observations responsive to the generated query comprises: identifying one or more candidate clusters of observations responsive to the generated query using the cluster index. 19. The computer storage medium of claim 17 , wherein generating a respective score for one or more of the candidate clusters comprises: generating a respective score for one or more of the candidate clusters using the cluster index.