| Year | Value | | 2021 | 11.25641 | | 2020 | 7.144167272 | | 2019 | 13.107 | | 2018 | 10.73322 | | 2017 | 13.765785 | | 2016 | 13.155752 | | 2015 | 13.873884 | | 2014 | 16.40872 | | 2013 | 15.65233 | | 2012 | 17.065 | | 2011 | 14.662 | | 2010 | 10.967098406736 | | 2009 | 15.291 | | 2008 | 18.439 | | 2007 | 21.987 | | 2006 | 20.176 | | 2005 | 20.8 | | 2004 | 23.83 | | 2003 | 4.797 | | 2002 | 11.6 | | 2001 | 46.884 | | 2000 | 31.157 | | 1999 | 21.8999996185 | | 1998 | 47 | | 1997 | 19.2999992371 | | 1996 | 15.8000001907 | | 1995 | 15 | | 1994 | 14.3000001907 | | 1993 | 19.8999996185 | | 1992 | 15.8000001907 | | 1991 | 5.5999999046 | | 1990 | 4.5999999046 | | 1989 | 2.5 | | 1988 | 6.5 | | 1987 | 3 | | 1986 | 2.5999999046 | | 1984 | 4 | | 1983 | 7.3000001907 | | 1982 | 10.3999996185 | | 1981 | 12.8000001907 | | 1980 | 17.2000007629 | | 1979 | 27.1000003815 | | 1978 | 27.2999992371 | | 1977 | 24.5 | | 1976 | 14 | | 1975 | 14 | | 1974 | 14 | | 1973 | 14 | | 1972 | 14.1000003815 | | 1971 | 13.6000003815 | | 1970 | 11.3999996185 |
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