Winkelmann, Rainer, Curtis Signorino, and Gary King. 1995. A Correction for an Underdispersed Event Count Probability Distribution. Political Analysis: 215–228. Copy at http://j.mp/keZPGW
Abstract:We demonstrate that the expected value and variance commonly given for a well-known probability distribution are incorrect. We also provide corrected versions and report changes in a computer program to account for the known practical uses of this distribution.