| Year | Value | | 2021 | 2.77 | | 2020 | 3.05 | | 2019 | 3.22 | | 2018 | 3.47 | | 2016 | 3.81 | | 2014 | 4.19 | | 2013 | 4.34 | | 2012 | 4.4 | | 2011 | 4.54 | | 2010 | 4.53 | | 2009 | 4.54 | | 2008 | 5.9 | | 2007 | 6.04 | | 2006 | 6.14 | | 2005 | 6.98 | | 2004 | 6.34 | | 2003 | 6.18 | | 2002 | 7.03 | | 2001 | 6.04 | | 2000 | 5.02 | | 1999 | 5.75 | | 1998 | 6.41 | | 1997 | 8.08 | | 1996 | 11.67 | | 1995 | 9.7 |
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