| Year | Value | | 2019 | 4.264 | | 2018 | 4.215 | | 2017 | 4.129 | | 2016 | 4.013 | | 2015 | 3.934 | | 2014 | 3.872 | | 2013 | 3.842 | | 2012 | 3.831 | | 2011 | 3.785 | | 2010 | 3.724 | | 2009 | 3.648 | | 2008 | 3.573 | | 2007 | 3.447 | | 2006 | 3.411 | | 2005 | 3.324 | | 2004 | 3.22 | | 2003 | 3.078 | | 2002 | 3.035 | | 2001 | 2.932 | | 2000 | 2.912 | | 1999 | 2.884 | | 1998 | 2.858 | | 1997 | 2.78 | | 1996 | 2.703 | | 1995 | 2.682 | | 1994 | 2.639 | | 1993 | 2.613 | | 1992 | 2.596 | | 1991 | 2.429 | | 1990 | 2.494 |
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