| Year | Value | | 2021 | 43.038447181784 | | 2020 | 43.038447181784 | | 2019 | 42.769690182904 | | 2018 | 42.491601343785 | | 2017 | 42.213512504666 | | 2016 | 41.935423665547 | | 2015 | 41.657334826428 | | 2014 | 39.986002239642 | | 2013 | 39.408361328854 | | 2012 | 39.776035834267 | | 2011 | 40.078387458007 | | 2010 | 40.287420679358 | | 2009 | 40.776409107876 | | 2008 | 41.377379619261 | | 2007 | 41.508025382606 | | 2006 | 40.271101157148 | | 2005 | 42.57185516984 | | 2004 | 44.074281448302 | | 2003 | 46.183277342292 | | 2002 | 45.60078387458 | | 2001 | 45.018197088466 | | 2000 | 44.435610302352 | | 1999 | 43.853023516237 | | 1998 | 43.270436730123 | | 1997 | 42.687849944009 | | 1996 | 42.105263157895 | | 1995 | 42.105263157895 | | 1994 | 42.105263157895 | | 1993 | 41.825307950728 | | 1992 | 39.986935423665 | | 1991 | 39.986935423665 | | 1990 | 39.986935423665 | | 1989 | 39.986935423665 | | 1988 | 39.986935423665 | | 1987 | 35.321015304218 | | 1986 | 35.321015304218 | | 1985 | 35.321015304218 | | 1984 | 35.321015304218 | | 1983 | 29.106009705114 | | 1982 | 28.96603210153 | | 1981 | 28.620754012691 | | 1980 | 28.46211272863 | | 1979 | 28.144830160508 | | 1978 | 28.04217991788 | | 1977 | 27.930197835013 | | 1976 | 27.342291899963 | | 1975 | 26.950354609929 | | 1974 | 26.829040686823 | | 1973 | 26.530421799179 | | 1972 | 26.129152668906 | | 1971 | 25.821201941023 | | 1970 | 25.709219858156 | | 1969 | 25.58790593505 | | 1968 | 25.569242254573 | | 1967 | 25.457260171706 | | 1966 | 25.25195968645 | | 1965 | 25.139977603583 | | 1964 | 25.027995520717 | | 1963 | 24.91601343785 | | 1962 | 24.804031354983 | | 1961 | 24.692049272116 |
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