| Year | Value | | 2021 | 60.680251201333 | | 2020 | 61.006287941556 | | 2019 | 61.331596054632 | | 2018 | 61.077319530711 | | 2017 | 60.877118377281 | | 2016 | 60.563609822475 | | 2015 | 60.360206782812 | | 2014 | 60.12791036436 | | 2013 | 59.959623821592 | | 2012 | 59.65257136471 | | 2011 | 59.46176501805 | | 2010 | 59.078973449543 | | 2009 | 58.736168985807 | | 2008 | 58.401899670766 | | 2007 | 58.112072422865 | | 2006 | 57.86496280074 | | 2005 | 57.063269274477 | | 2004 | 57.026639825455 | | 2003 | 56.70188414716 | | 2002 | 56.382242557128 | | 2001 | 56.028605759122 | | 2000 | 55.780000752831 | | 1999 | 55.354708077818 | | 1998 | 54.921379001091 | | 1997 | 54.663740055967 | | 1996 | 54.603673911134 | | 1995 | 54.453501636878 | | 1994 | 54.692062210969 | | 1993 | 54.82672119222 | | 1992 | 55.020112294529 | | 1991 | 55.209073414096 | | 1990 | 55.358120166567 | | 1989 | 55.285415693842 | | 1988 | 55.166798390574 | | 1987 | 55.048355139189 | | 1986 | 55.139726618012 | | 1985 | 55.167492136045 | | 1984 | 55.163692816127 | | 1983 | 54.962503157879 | | 1982 | 54.612736273924 | | 1981 | 54.386905226634 | | 1980 | 53.857287975286 | | 1979 | 53.361608595411 | | 1978 | 53.020741519187 | | 1977 | 52.65900113866 | | 1976 | 52.287654432941 | | 1975 | 51.797138494962 | | 1974 | 51.307344219929 | | 1973 | 50.978253204908 | | 1972 | 50.521694244894 | | 1971 | 48.229644915204 | | 1970 | 49.489636352773 | | 1969 | 48.612700211183 | | 1968 | 48.047243679457 | | 1967 | 47.187981469289 | | 1966 | 47.087216320298 | | 1965 | 46.378842751684 | | 1964 | 45.817112430602 | | 1963 | 45.219446021443 | | 1962 | 44.534136792495 | | 1961 | 43.739127424013 | | 1960 | 43.131328914189 |
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