| Year | Value | | 2021 | 63.403288901811 | | 2020 | 63.748926085736 | | 2019 | 64.212698884332 | | 2018 | 63.88195354281 | | 2017 | 63.542837464741 | | 2016 | 63.167934891851 | | 2015 | 62.754896764892 | | 2014 | 62.422434930703 | | 2013 | 62.110370611718 | | 2012 | 61.777333689065 | | 2011 | 61.462849415428 | | 2010 | 60.906850340328 | | 2009 | 60.460941272198 | | 2008 | 59.78298105943 | | 2007 | 59.600649451745 | | 2006 | 59.248547787191 | | 2005 | 58.610570301919 | | 2004 | 58.188952039391 | | 2003 | 57.867306738238 | | 2002 | 57.424565899583 | | 2001 | 57.016728684086 | | 2000 | 56.635870269043 | | 1999 | 56.190550516356 | | 1998 | 55.247936170363 | | 1997 | 55.006932916867 | | 1996 | 54.561232535355 | | 1995 | 54.423076954256 | | 1994 | 54.225244509098 | | 1993 | 53.876851235788 | | 1992 | 53.629493950962 | | 1991 | 53.307249529897 | | 1990 | 53.540560649288 | | 1989 | 53.310110968359 | | 1988 | 52.672773373511 | | 1987 | 52.711169468158 | | 1986 | 52.353868939063 | | 1985 | 51.933917903654 | | 1984 | 51.690722150555 | | 1983 | 51.553316170556 | | 1982 | 51.757590035492 | | 1981 | 51.531299614156 | | 1980 | 51.151715002824 | | 1979 | 50.646055592379 | | 1978 | 50.218208448535 | | 1977 | 49.848357630461 | | 1976 | 49.268101253754 | | 1975 | 48.691352379423 | | 1974 | 48.506517400405 | | 1973 | 48.398859613681 | | 1972 | 47.86886879215 | | 1971 | 44.110011465773 | | 1970 | 46.406109877701 | | 1969 | 46.893737950545 | | 1968 | 46.542852863288 | | 1967 | 46.026363001909 | | 1966 | 45.596313830764 | | 1965 | 45.111826307485 | | 1964 | 44.924641244422 | | 1963 | 44.263890054087 | | 1962 | 43.946983205966 | | 1961 | 43.359497472991 | | 1960 | 42.845919869226 |
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