| Year | Value | | 2021 | 3.815 | | 2020 | 3.489 | | 2019 | 3.225 | | 2018 | 3.317 | | 2017 | 3.26 | | 2016 | 3.302 | | 2015 | 3.304 | | 2014 | 3.829 | | 2013 | 3.309 | | 2012 | 3.436 | | 2011 | 3.459 | | 2010 | 3.484 | | 2009 | 3.554 | | 2008 | 3.566 | | 2007 | 3.675 | | 2006 | 3.9 | | 2005 | 4 | | 2004 | 4.1 | | 2003 | 4.2 | | 2002 | 4.3 | | 2001 | 4.4 | | 2000 | 4.5 | | 1999 | 4.6 | | 1998 | 4.8 | | 1997 | 4.9 | | 1996 | 5.16 | | 1995 | 5.42 | | 1994 | 5.68 | | 1993 | 5.94 | | 1992 | 6.2 | | 1991 | 6.46 | | 1990 | 6.72 |
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