A pitfall in the use of extended likelihood for fitting fractions of   pure samples in mixed samples
  The paper elucidates, with an analytic example, a subtle mistake in the application of the extended likelihood method to the problem of determining the fractions of pure samples in a mixed sample from the shape of the distribution of a random variable. This mistake, which affects two widely used software packages, leads to a misestimate of the errors. 
A. Nappi