2David Ray Anderson, Dennis J. Sweeney, Thomas Arthur Williams. Statistics for Business and Economics. McGraw--Hill, 2009.
3Expected Mail Volumes VS. Actual Mail Volumes, Postal Solutions, http: //www. postal-- solutions, corn, 2011--02--10.
4Mehdi Farsi, Massimo Filipini, UrsTrinkner. Economies of Scale, Density and Scope in Swiss Post's Mail Delivery in Liberaliza- tion of the postal and delivery sector Edited by Miehael A. Crew, Paul R. Kleindorfer. Edward Elgar Publishing, Inc, 2006.