| Year | Value | | 2021 | 17.307683 | | 2020 | 13.689186 | | 2019 | 5.847551 | | 2018 | 8.84572 | | 2017 | 4.5668399023 | | 2016 | 4.895080058 | | 2015 | 4.536371616 | | 2014 | 4.5996 | | 2013 | 5.759868 | | 2012 | 5.7623749310587 | | 2011 | 6.486564880203 | | 2010 | 6.463516539 | | 2009 | 6.282 | | 2008 | 7.233 | | 2007 | 7.643 | | 2006 | 7.212 | | 2005 | 6.936 | | 2004 | 6.73 | | 2003 | 5.751 | | 2002 | 5.477 | | 2001 | 16.178 | | 2000 | 16.997 | | 1999 | 16 | | 1998 | 15.6000003815 | | 1997 | 17.8999996185 | | 1996 | 17.8999996185 | | 1995 | 16.8999996185 | | 1994 | 16 | | 1993 | 18.2999992371 | | 1992 | 13.8000001907 | | 1991 | 23.8999996185 | | 1990 | 11.1999998093 | | 1989 | 10.5 | | 1988 | 27.8999996185 | | 1987 | 5.9000000954 | | 1986 | 6.1999998093 | | 1985 | 6.6999998093 | | 1984 | 4.5999999046 | | 1983 | 3.9000000954 | | 1982 | 2.7000000477 | | 1981 | 2.9000000954 | | 1980 | 2.5 | | 1979 | 1.7999999523 | | 1978 | 1.6000000238 | | 1977 | 1.5 | | 1976 | 1.3999999762 | | 1975 | 2 | | 1974 | 0.6000000238 | | 1973 | 0.5 | | 1972 | 0.5 | | 1971 | 0.400000006 | | 1970 | 0.400000006 |
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