| Year | Value | | 2019 | 5.58 | | 2018 | 5.62 | | 2017 | 5.66 | | 2016 | 5.76 | | 2015 | 5.83 | | 2014 | 5.85 | | 2013 | 5.93 | | 2012 | 5.99 | | 2011 | 6.05 | | 2010 | 6.12 | | 2009 | 6.15 | | 2008 | 6.25 | | 2007 | 6.27 | | 2006 | 6.35 | | 2005 | 6.44 | | 2004 | 6.48 | | 2003 | 6.53 | | 2002 | 6.54 | | 2001 | 6.62 | | 2000 | 6.72 | | 1999 | 7.0999999046 | | 1998 | 7.1999998093 | | 1997 | 7.1999998093 | | 1994 | 7.5999999046 | | 1993 | 7.6999998093 | | 1992 | 7.6999998093 | | 1991 | 7.9000000954 | | 1990 | 8 | | 1985 | 9.1000003814697 | | 1980 | 9.3999996185303 | | 1970 | 8.3000001907349 |
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