@misc{Maddison2009,
author = {Maddison, A.},
title= {Historical statistics of the world economy: 1–2008 AD},
year = {2009},
URL = {www.ggdc.net/maddison/Historical_Statistics/horizontal-file_02-2010.xls},
publisher = {University of Groningen, Maddison Project},
address = {Groningen},
}