Testing Consistency of Two Histograms
  Several approaches to testing the hypothesis that two histograms are drawn from the same distribution are investigated. We note that single-sample continuous distribution tests may be adapted to this two-sample grouped data situation. The difficulty of not having a fully-specified null hypothesis is an important consideration in the general case, and care is required in estimating probabilities with ``toy'' Monte Carlo simulations. The performance of several common tests is compared; no single test performs best in all situations. 
Frank C. Porter