| Year | Value | | 2021 | 2.06 | | 2020 | 1.87 | | 2019 | 2.1 | | 2018 | 1.99 | | 2017 | 1.99 | | 2016 | 2.95 | | 2015 | 3.14 | | 2014 | 3.06 | | 2013 | 3.07 | | 2012 | 3.23 | | 2011 | 3.37 | | 2010 | 3.43 | | 2009 | 4.08 | | 2008 | 4.14 | | 2007 | 4.14 | | 2006 | 4.16 | | 2005 | 4.29 | | 2004 | 4.28 | | 2003 | 6.38 | | 2002 | 4.82 | | 2001 | 5.17 | | 2000 | 4.72 | | 1999 | 5.05 | | 1998 | 5.16 | | 1997 | 6.74 | | 1996 | 7.82 | | 1995 | 9.28 | | 1993 | 8.92 | | 1989 | 9.66 |
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