| Year | Value | | 2018 | 2.97 | | 2017 | 2.97 | | 2016 | 2.96 | | 2015 | 2.92 | | 2014 | 2.57 | | 2013 | 2.56 | | 2012 | 2.54 | | 2011 | 2.62 | | 2010 | 2.73 | | 2009 | 2.83 | | 2008 | 4.85 | | 2007 | 5.1 | | 2006 | 5.26 | | 2005 | 5.46 | | 2004 | 5.64 | | 2003 | 5.67 | | 2002 | 5.78 | | 2001 | 5.86 | | 2000 | 6.13 | | 1999 | 4.6999998093 | | 1998 | 4.8000001907 | | 1997 | 4.9000000954 | | 1996 | 5.0999999046 | | 1995 | 5.1999998093 | | 1994 | 5.3000001907 | | 1993 | 5.5 | | 1992 | 5.6999998093 | | 1991 | 6 | | 1990 | 6.0999999046 | | 1989 | 6.1999998092651 | | 1988 | 6.4000000953674 | | 1987 | 7.1999998092651 | | 1986 | 7.9000000953674 | | 1985 | 8.1999998092651 | | 1984 | 8.3999996185303 | | 1983 | 8.6000003814697 | | 1982 | 8.8000001907349 | | 1981 | 9 | | 1980 | 9 |
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