| Year | Value | | 2021 | 3.28 | | 2020 | 3.41 | | 2019 | 3.37 | | 2018 | 3.47 | | 2017 | 3.69 | | 2015 | 3.83 | | 2014 | 4 | | 2013 | 4.29 | | 2012 | 4.1 | | 2011 | 5.18 | | 2010 | 5.03 | | 2009 | 3.73 | | 2008 | 3.82 | | 2007 | 5.05 | | 2006 | 5.04 | | 2005 | 6.68 | | 2004 | 5.99 | | 2003 | 7.21 | | 2002 | 5.76 | | 2001 | 7.47 | | 2000 | 7.38 | | 1999 | 7.03 | | 1998 | 5.94 | | 1997 | 8.1 | | 1996 | 8.71 | | 1995 | 10.04 |
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