EM algorithm and variants: an informal tutorial
  The expectation-maximization (EM) algorithm introduced by Dempster et al in 1977 is a very general method to solve maximum likelihood estimation problems. In this informal report, we review the theory behind EM as well as a number of EM variants, suggesting that beyond the current state of the art is an even much wider territory still to be discovered. 
Alexis Roche
stat.CO