| Year | Value | | 2021 | 2.165 | | 2020 | 2.036 | | 2019 | 1.926 | | 2018 | 1.849 | | 2017 | 1.827 | | 2016 | 1.788 | | 2015 | 1.774 | | 2014 | 1.736 | | 2013 | 1.747 | | 2012 | 1.724 | | 2011 | 1.699 | | 2010 | 1.687 | | 2009 | 1.643 | | 2008 | 1.587 | | 2007 | 1.538 | | 2006 | 1.501 | | 2005 | 1.468 | | 2004 | 1.433 | | 2003 | 1.413 | | 2002 | 1.393 | | 2001 | 1.38 | | 2000 | 1.33 | | 1999 | 1.261 | | 1998 | 1.206 | | 1997 | 1.168 | | 1996 | 1.146 | | 1995 | 1.144 | | 1994 | 1.129 | | 1993 | 1.066 | | 1992 | 1.012 | | 1991 | 0.985 | | 1990 | 0.932 |
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