| Year | Value | | 2021 | 58.065903754861 | | 2020 | 58.490235060487 | | 2019 | 59.033321206702 | | 2018 | 58.696319938773 | | 2017 | 58.272973900142 | | 2016 | 57.90489880073 | | 2015 | 57.525862238708 | | 2014 | 57.154581699155 | | 2013 | 56.773856982813 | | 2012 | 56.38005942626 | | 2011 | 55.921944377805 | | 2010 | 55.381208936994 | | 2009 | 54.90396768617 | | 2008 | 54.195745849748 | | 2007 | 53.600317403774 | | 2006 | 53.093699796811 | | 2005 | 52.46866210428 | | 2004 | 51.871380843547 | | 2003 | 51.410120441239 | | 2002 | 50.909662308314 | | 2001 | 50.427850121262 | | 2000 | 49.950387097497 | | 1999 | 49.005259558798 | | 1998 | 47.837786538865 | | 1997 | 48.249585766519 | | 1996 | 47.957149069248 | | 1995 | 47.954715851034 | | 1994 | 48.161255621932 | | 1993 | 47.177116579404 | | 1992 | 46.689567860649 | | 1991 | 46.788764469119 | | 1990 | 46.772568093733 | | 1989 | 46.777373987815 | | 1988 | 45.448367925953 | | 1987 | 45.888082856182 | | 1986 | 46.452666397171 | | 1985 | 45.903295372628 | | 1984 | 45.758546630668 | | 1983 | 46.117784766969 | | 1982 | 46.382617039254 | | 1981 | 46.263731611484 | | 1980 | 46.087302904861 | | 1979 | 46.101823157679 | | 1978 | 45.770363127424 | | 1977 | 45.572563843077 | | 1976 | 45.080240181396 | | 1975 | 44.490134120793 | | 1974 | 44.03253762037 | | 1973 | 43.639895162552 | | 1972 | 42.6771168098 | | 1971 | 42.379248184768 | | 1970 | 41.757084965406 | | 1969 | 41.069427428009 | | 1968 | 40.77434259523 | | 1967 | 40.236628949039 | | 1966 | 40.347435829351 | | 1965 | 40.068472029673 | | 1964 | 39.920103793864 | | 1963 | 39.499198512212 | | 1962 | 39.29319711337 | | 1961 | 38.656089938184 | | 1960 | 38.439181195769 |
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