| Year | Value | | 2019 | 2.46 | | 2018 | 2.5 | | 2017 | 2.54 | | 2016 | 2.57 | | 2015 | 2.61 | | 2014 | 2.73 | | 2013 | 2.76 | | 2012 | 2.81 | | 2011 | 2.88 | | 2010 | 2.93 | | 2009 | 3.26 | | 2008 | 3.33 | | 2007 | 3.39 | | 2006 | 3.54 | | 2005 | 3.72 | | 2004 | 3.86 | | 2003 | 3.95 | | 2002 | 3.98 | | 2001 | 4.04 | | 2000 | 4.08 | | 1999 | 4.1999998093 | | 1998 | 4.4000000954 | | 1997 | 4.5999999046 | | 1996 | 4.5999999046 | | 1995 | 4.8000001907 | | 1994 | 4.8000001907 | | 1993 | 5.0999999046 | | 1992 | 5.4000000954 | | 1991 | 5.5999999046 | | 1990 | 5.9000000954 | | 1985 | 7.4000000953674 | | 1980 | 8.1000003814697 | | 1970 | 9.6000003814697 | | 1960 | 10.699999809265 |
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