| Year | Value | | 2017 | 3.57 | | 2016 | 3.94 | | 2015 | 4.22 | | 2014 | 5.5 | | 2013 | 5.5 | | 2012 | 5.34 | | 2011 | 4.78 | | 2010 | 4.71 | | 2009 | 5.42 | | 2008 | 5.75 | | 2007 | 6.26 | | 2006 | 4.97 | | 2005 | 7.3 | | 2004 | 6.38 | | 2003 | 4.93 | | 2002 | 8.12 | | 2001 | 8.13 | | 2000 | 7.76 | | 1999 | 8.19 | | 1998 | 6.43 | | 1997 | 9.38 | | 1996 | 8.58 | | 1995 | 6.74 | | 1994 | 11.11 | | 1993 | 8.01 | | 1991 | 19.08 |
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