| Year | Value | | 2021 | 67.306948633356 | | 2020 | 67.521909093455 | | 2019 | 67.432679948287 | | 2018 | 67.339447376324 | | 2017 | 67.238171137044 | | 2016 | 66.946707244007 | | 2015 | 66.629602668168 | | 2014 | 66.729087695134 | | 2013 | 66.702043504951 | | 2012 | 66.691894357706 | | 2011 | 66.571968411746 | | 2010 | 66.44975243774 | | 2009 | 66.095659356739 | | 2008 | 66.159110268139 | | 2007 | 66.019258359649 | | 2006 | 65.998890542666 | | 2005 | 65.784014698519 | | 2004 | 65.619052744032 | | 2003 | 65.411478715963 | | 2002 | 65.071738571507 | | 2001 | 65.219619195129 | | 2000 | 65.05558104112 | | 1999 | 65.049685020641 | | 1998 | 64.947623797384 | | 1997 | 64.756387237379 | | 1996 | 64.599688150655 | | 1995 | 64.352678894578 | | 1994 | 64.093787001018 | | 1993 | 63.883736260481 | | 1992 | 63.662662700463 | | 1991 | 63.430495535083 | | 1990 | 63.186821140468 | | 1989 | 62.92572649562 | | 1988 | 62.731665765365 | | 1987 | 62.412163771195 | | 1986 | 62.124557092197 | | 1985 | 62.03484898801 | | 1984 | 61.615391308601 | | 1983 | 61.208423976608 | | 1982 | 60.843304219065 | | 1981 | 60.662981914677 | | 1980 | 60.451616825778 | | 1979 | 60.30070084286 | | 1978 | 59.947063671712 | | 1977 | 59.542449917827 | | 1976 | 59.322712359627 | | 1975 | 58.815333062911 | | 1974 | 58.803559247184 | | 1973 | 58.234223887982 | | 1972 | 58.316643944239 | | 1971 | 58.116482529405 | | 1970 | 57.801569115062 | | 1969 | 57.448374444836 | | 1968 | 57.318940052956 | | 1967 | 56.966812515524 | | 1966 | 56.470860943702 | | 1965 | 56.091173778036 | | 1964 | 55.176964349646 | | 1963 | 55.000049889682 | | 1962 | 54.470266685623 | | 1961 | 53.883117949029 | | 1960 | 53.414629318894 |
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