| Year | Value | | 2019 | 4.26 | | 2018 | 4.51 | | 2017 | 4.66 | | 2016 | 4.81 | | 2015 | 4.93 | | 2014 | 5.05 | | 2013 | 5.17 | | 2012 | 5.15 | | 2011 | 5.28 | | 2010 | 5.37 | | 2009 | 5.47 | | 2008 | 5.57 | | 2007 | 5.67 | | 2006 | 5.65 | | 2005 | 5.79 | | 2004 | 6.39 | | 2003 | 6 | | 2002 | 6 | | 2001 | 6.0999999046 | | 2000 | 6.1999998093 | | 1999 | 6.3000001907 | | 1998 | 6.8000001907 | | 1997 | 6.9000000954 | | 1996 | 7 | | 1995 | 7.0999999046 | | 1994 | 7.1999998093 | | 1993 | 7.4000000954 | | 1992 | 7.3000001907 | | 1991 | 7.5 | | 1990 | 7.5999999046 | | 1989 | 7.8000001907349 | | 1988 | 7.9000000953674 | | 1987 | 8 | | 1986 | 8.1000003814697 |
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