| Year | Value | | 2021 | 1.93 | | 2020 | 1.97 | | 2019 | 2.03 | | 2018 | 2.22 | | 2017 | 2.23 | | 2016 | 2.3 | | 2015 | 2.5 | | 2014 | 2.84 | | 2013 | 2.77 | | 2012 | 2.76 | | 2011 | 2.8 | | 2010 | 2.85 | | 2009 | 4.05 | | 2008 | 4.06 | | 2007 | 3.91 | | 2006 | 3.83 | | 2005 | 4.27 | | 2004 | 5.19 | | 2003 | 5.12 | | 2002 | 5.1 | | 2001 | 5.37 | | 2000 | 6.55 | | 1999 | 6.74 | | 1998 | 6.99 | | 1997 | 7.63 | | 1996 | 7.75 | | 1993 | 9.67 | | 1991 | 20.1 |
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