| Year | Value | | 2021 | 4.2036124794746 | | 2020 | 4.2036124794746 | | 2019 | 4.1488779419814 | | 2018 | 4.072249589491 | | 2017 | 3.9956212370006 | | 2016 | 3.9189928845101 | | 2015 | 3.8423645320197 | | 2014 | 3.7657361795293 | | 2013 | 3.6891078270389 | | 2012 | 3.6124794745484 | | 2011 | 3.535851122058 | | 2010 | 3.4592227695676 | | 2009 | 3.3825944170772 | | 2008 | 3.7985769020252 | | 2007 | 4.1160372194855 | | 2006 | 4.4334975369458 | | 2005 | 4.7509578544061 | | 2004 | 5.0684181718665 | | 2003 | 5.3858784893268 | | 2002 | 5.7033388067871 | | 2001 | 6.0207991242474 | | 2000 | 6.3382594417077 | | 1999 | 6.655719759168 | | 1998 | 6.863711001642 | | 1997 | 7.1811713191023 | | 1996 | 7.4986316365627 | | 1995 | 7.816091954023 | | 1994 | 8.4072249589491 | | 1993 | 8.7246852764094 | | 1992 | 9.0421455938697 | | 1991 | 9.35960591133 | | 1990 | 8.7575259989053 | | 1989 | 8.2101806239737 | | 1988 | 7.6628352490421 | | 1987 | 7.1154898741106 | | 1986 | 6.568144499179 | | 1985 | 6.568144499179 | | 1984 | 6.0207991242474 | | 1983 | 5.7471264367816 | | 1982 | 5.4734537493158 | | 1981 | 5.19978106185 | | 1980 | 4.9261083743842 | | 1979 | 4.9261083743842 | | 1978 | 4.9261083743842 | | 1977 | 4.6524356869184 | | 1976 | 4.3787629994526 | | 1975 | 4.1050903119869 | | 1974 | 3.8314176245211 | | 1973 | 3.8314176245211 | | 1972 | 3.8314176245211 | | 1971 | 3.6672140120416 | | 1970 | 3.6672140120416 | | 1969 | 3.6672140120416 | | 1968 | 3.6672140120416 | | 1967 | 4.3240284619595 | | 1966 | 4.4334975369458 | | 1965 | 4.4334975369458 | | 1964 | 4.7071702244116 | | 1963 | 4.7071702244116 | | 1962 | 4.8713738368911 | | 1961 | 4.8713738368911 |
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