http://blog.daum.net/kori2sal/7087262
참고자료
http://www.ggdc.net/maddison/articles/ruggles.pdf
http://www2.ttcn.ne.jp/honkawa/4545.html
http://www.ggdc.net/maddison/Historical_Statistics/horizontal-file_03-2009.xls
http://www.ggdc.net/maddison/Historical_Statistics/vertical-file_03-2009.xls