| Year | Value | | 2021 | 1.6239 | | 2020 | 0.889334 | | 2019 | 3.334 | | 2018 | 2.71 | | 2017 | 2.8045 | | 2016 | 4.2237 | | 2015 | 4.7179 | | 2014 | 5.2917 | | 2013 | 5.25614 | | 2012 | 5.622633 | | 2011 | 6.0952 | | 2010 | 5.163786 | | 2009 | 3.987 | | 2008 | 6.421 | | 2007 | 5.684 | | 2006 | 4.984 | | 2005 | 5.161 | | 2004 | 4.881 | | 2003 | 6.797 | | 2002 | 8.664 | | 2001 | 9.152 | | 2000 | 12.102 | | 1999 | 11.6999998093 | | 1998 | 12.1000003815 | | 1997 | 12.5 | | 1996 | 14.8999996185 | | 1995 | 17.8999996185 | | 1994 | 18.6000003815 | | 1993 | 15.1000003815 | | 1992 | 11 | | 1991 | 13.3999996185 | | 1990 | 12.8000001907 | | 1989 | 14.5 | | 1988 | 12.8000001907 | | 1987 | 11.6000003815 | | 1986 | 10.1000003815 | | 1985 | 9.6999998093 | | 1984 | 11.1999998093 | | 1983 | 6.0999999046 | | 1982 | 10 | | 1981 | 11.3000001907 | | 1980 | 12.3000001907 | | 1979 | 14.3000001907 | | 1978 | 13.1999998093 | | 1977 | 13.6999998093 | | 1976 | 10.6000003815 | | 1975 | 8.8000001907 | | 1974 | 6.6999998093 | | 1973 | 5.9000000954 | | 1972 | 5.5999999046 | | 1971 | 6.3000001907 | | 1970 | 7.0999999046 |
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