| Year | Value | | 2021 | 58.26433575251 | | 2020 | 58.902879810441 | | 2019 | 59.364417392891 | | 2018 | 59.010982135034 | | 2017 | 58.657879819471 | | 2016 | 58.262184315743 | | 2015 | 57.766494868014 | | 2014 | 57.36744372359 | | 2013 | 56.942175398371 | | 2012 | 56.494535889157 | | 2011 | 55.967547393212 | | 2010 | 55.326206642418 | | 2009 | 54.729215121907 | | 2008 | 54.043045013449 | | 2007 | 53.463047694644 | | 2006 | 52.869201559909 | | 2005 | 52.115474306335 | | 2004 | 51.510935554225 | | 2003 | 51.013229199473 | | 2002 | 50.489156259301 | | 2001 | 50.123520934171 | | 2000 | 49.766670945712 | | 1999 | 48.871035366398 | | 1998 | 48.093475547287 | | 1997 | 48.420952196218 | | 1996 | 48.207112631755 | | 1995 | 48.253113078093 | | 1994 | 48.171677255328 | | 1993 | 47.700403984787 | | 1992 | 47.500252054099 | | 1991 | 47.746964067714 | | 1990 | 47.797759872913 | | 1989 | 47.930431828677 | | 1988 | 46.998559426373 | | 1987 | 47.336267388343 | | 1986 | 47.124157138231 | | 1985 | 46.753686315314 | | 1984 | 46.523234345851 | | 1983 | 46.589131545147 | | 1982 | 47.43363605728 | | 1981 | 47.223478778277 | | 1980 | 46.883731302134 | | 1979 | 46.51209710182 | | 1978 | 46.024703404197 | | 1977 | 45.80733813019 | | 1976 | 45.405262947701 | | 1975 | 44.817958491904 | | 1974 | 44.432481129887 | | 1973 | 44.211278705831 | | 1972 | 43.417323191668 | | 1971 | 43.296874985588 | | 1970 | 42.804827774354 | | 1969 | 42.248357847764 | | 1968 | 42.010892654184 | | 1967 | 41.563347308286 | | 1966 | 41.470090133684 | | 1965 | 41.471011409073 | | 1964 | 41.338801166199 | | 1963 | 40.921327142313 | | 1962 | 40.756156123974 | | 1961 | 40.193582924116 | | 1960 | 39.936486093579 |
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