| Year | Value | | 2021 | 3.49 | | 2020 | 3.27 | | 2019 | 3.24 | | 2018 | 3.51 | | 2017 | 3.62 | | 2016 | 3.69 | | 2015 | 3.94 | | 2014 | 3.84 | | 2013 | 4.12 | | 2012 | 4.23 | | 2011 | 4.13 | | 2010 | 4.02 | | 2009 | 3.4 | | 2008 | 3.54 | | 2007 | 4.32 | | 2006 | 4.1 | | 2005 | 4.39 | | 2004 | 4.19 | | 2003 | 4.08 | | 2002 | 4.42 | | 2001 | 5.07 | | 2000 | 6.5 | | 1999 | 4.47 | | 1998 | 6.92 | | 1997 | 8.34 | | 1996 | 4.66 | | 1995 | 10.24 | | 1993 | 2.86 | | 1988 | 2.1 |
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