Clustering in Highest Energy Cosmic Rays: Physics or Statistics?
  Directional clustering can be expected in cosmic ray observations due to purely statistical fluctuations for sources distributed randomly in the sky. We develop an analytic approach to estimate the probability of random cluster configurations, and use these results to study the strong potential of the HiRes, Auger, Telescope Array and EUSO/OWL/AirWatch facilities for deciding whether any observed clustering is most likely due to non-random sources. 
Haim Goldberg and Thomas J. Weiler