| Year | Value | | 2021 | 2.6608232368733 | | 2020 | 2.6957974008123 | | 2019 | 2.737865274637 | | 2018 | 2.8263275326116 | | 2017 | 2.8835908982843 | | 2016 | 2.919411415663 | | 2015 | 3.0070891858051 | | 2014 | 3.0287944504514 | | 2013 | 3.0194158123411 | | 2012 | 2.9920963438094 | | 2011 | 2.9495134216244 | | 2010 | 2.9206142886271 | | 2009 | 2.8894779590249 | | 2008 | 2.8850379542553 | | 2007 | 2.891116981339 | | 2006 | 2.8965531516409 | | 2005 | 2.916322155343 | | 2004 | 2.9444382408314 | | 2003 | 2.9839840991418 | | 2002 | 3.0240844647532 | | 2001 | 3.096427526387 | | 2000 | 3.1648846474377 | | 1999 | 3.2523011938157 | | 1998 | 3.3568222009926 | | 1997 | 3.4708468643107 | | 1996 | 3.6208082683915 | | 1995 | 3.8306455331745 | | 1994 | 4.0582570460605 | | 1993 | 4.2662231871623 | | 1992 | 4.476993187284 | | 1991 | 4.6622000081913 | | 1990 | 4.8648794573118 | | 1989 | 5.0558057549194 | | 1988 | 5.2571214380475 | | 1987 | 5.4512158524836 | | 1986 | 5.6395094675131 | | 1985 | 5.8120196415119 | | 1984 | 5.947359612649 | | 1983 | 6.0577282703348 | | 1982 | 6.1428459743848 | | 1981 | 6.2122815256038 | | 1980 | 6.2943067798311 | | 1979 | 6.3582514449469 | | 1978 | 6.3434349197448 | | 1977 | 6.3614291114269 | | 1976 | 6.3958465745612 | | 1975 | 6.4143445139151 | | 1974 | 6.4602463351176 | | 1973 | 6.5160609854612 | | 1972 | 6.5984245273192 | | 1971 | 6.699814444759 | | 1970 | 6.7917062946552 | | 1969 | 6.8717684609275 | | 1968 | 6.9218126341245 | | 1967 | 6.9685697721357 | | 1966 | 7.0022619584841 | | 1965 | 7.0342711891645 | | 1964 | 7.071407047139 | | 1963 | 7.0975307493736 | | 1962 | 7.1177354554624 | | 1961 | 7.0335363469549 | | 1960 | 7.0025553238903 |
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