Steiner, S. H., Cook, R. J., and Farewell, V. T. (1999), Monitoring
  paired binary surgical outcomes using cumulative sum charts,
  Statistics in Medicine, 18, pp. 69--86.
  
De Leval, Marc R., Franiois, K., Bull, C., Brawn, W. B. and
  Spiegelhalter, D. (1994),  Analysis of a cluster of
  surgical failures, Journal of Thoracic and Cardiovascular Surgery,
  March, pp. 914--924.