| Year | Value | | 2021 | 6.338 | | 2019 | 4.83 | | 2018 | 4.083 | | 2016 | 2.286 | | 2015 | 3.566 | | 2014 | 1.827 | | 2013 | 2.954 | | 2012 | 3.667 | | 2011 | 0.796 | | 2010 | 0.653 | | 2009 | 0.535 | | 2008 | 0.423 | | 2007 | 0.398 | | 2006 | 0.582 | | 2005 | 7.05 | | 2004 | 7.4 | | 2003 | 7.49 | | 2002 | 7.83 | | 2001 | 6.88 | | 2000 | 7.16 | | 1999 | 5.35 | | 1998 | 5.7 | | 1997 | 5.81 | | 1996 | 4.79 | | 1995 | 5.03 | | 1994 | 4.26 | | 1993 | 4.28 | | 1992 | 5.18 | | 1991 | 5.85 | | 1990 | 1.97 | | 1989 | 2.03 | | 1988 | 2.16 | | 1987 | 3.05 | | 1986 | 3.97 | | 1985 | 3.97 | | 1984 | 4.16 | | 1983 | 4.3 | | 1982 | 3.82 | | 1981 | 3.82 | | 1980 | 4.01 | | 1979 | 4.12 | | 1978 | 4.18 | | 1977 | 1.9 | | 1976 | 1.9 | | 1975 | 1.7 | | 1974 | 1.67 | | 1973 | 2 | | 1972 | 2.15 | | 1971 | 2.09 |
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