| Year | Value | | 2021 | 1.56490641 | | 2020 | 1.683881 | | 2019 | 1.84341946 | | 2018 | 19.792972067372 | | 2017 | 22.001546067558 | | 2016 | 24.161930857 | | 2015 | 24.801963933925 | | 2014 | 12.198352181143 | | 2013 | 12.198352181143 | | 2009 | 7.966 | | 2008 | 10.036 | | 2007 | 10.036 | | 2006 | 11.269 | | 2005 | 10.029 | | 2004 | 9.809 | | 2003 | 9.996 | | 2002 | 8.724 | | 2001 | 2.792 | | 2000 | 8.824 | | 1999 | 9.6999998093 | | 1998 | 27 | | 1997 | 5 | | 1996 | 4.6999998093 | | 1995 | 1.8999999762 | | 1994 | 7.1999998093 | | 1993 | 9.8000001907 | | 1992 | 10.3000001907 | | 1991 | 27.5 | | 1990 | 23.5 | | 1989 | 17 | | 1988 | 29.5 | | 1987 | 37.2000007629 | | 1986 | 32.4000015259 | | 1985 | 28.7000007629 | | 1984 | 30.5 | | 1983 | 33.2999992371 | | 1982 | 25.5 | | 1981 | 15.6999998093 | | 1980 | 9.3000001907 | | 1979 | 4.5 | | 1978 | 3.4000000954 | | 1977 | 4.0999999046 | | 1976 | 3.5999999046 | | 1975 | 4.3000001907 | | 1974 | 6.6999998093 | | 1973 | 7.1999998093 | | 1972 | 7 | | 1971 | 6.5 | | 1970 | 5.3000001907 |
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