| Year | Value | | 2021 | 2.629 | | 2020 | 2.423532 | | 2019 | 6.347 | | 2018 | 4.775 | | 2017 | 3.557 | | 2016 | 4.37 | | 2015 | 4.8636616592381 | | 2014 | 5.3665029910009 | | 2013 | 5.882496 | | 2012 | 4.9223802365457 | | 2011 | 8.075 | | 2010 | 6.14 | | 2009 | 5.827 | | 2008 | 6.743 | | 2007 | 5.778 | | 2006 | 5.196 | | 2005 | 4.928 | | 2004 | 5.344 | | 2003 | 6.627 | | 2002 | 7.095 | | 2001 | 6.885 | | 2000 | 7.164 | | 1999 | 6.5999999046 | | 1998 | 5.9000000954 | | 1997 | 5.1999998093 | | 1996 | 5.4000000954 | | 1995 | 5.5 | | 1994 | 11.3000001907 | | 1993 | 9.8999996185 | | 1992 | 8.6000003815 | | 1991 | 9.3999996185 | | 1990 | 9.1000003815 | | 1989 | 10.1000003815 | | 1988 | 10.1999998093 | | 1987 | 8.6000003815 | | 1986 | 10.3000001907 | | 1985 | 9.1000003815 | | 1984 | 12.3999996185 | | 1983 | 15.3000001907 | | 1982 | 10.3999996185 | | 1981 | 9.1999998093 | | 1980 | 8.6999998093 | | 1979 | 9.6999998093 | | 1978 | 7.1999998093 | | 1977 | 5.4000000954 | | 1976 | 5.3000001907 | | 1975 | 4.9000000954 |
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