| Year | Value | | 2017 | 3.77 | | 2016 | 3.96 | | 2015 | 4.2 | | 2014 | 5 | | 2013 | 4.78 | | 2012 | 4.86 | | 2011 | 4.44 | | 2010 | 4.24 | | 2009 | 4.71 | | 2008 | 4.9 | | 2007 | 5.22 | | 2006 | 4.67 | | 2005 | 6.14 | | 2004 | 5.28 | | 2003 | 4.6 | | 2002 | 6.33 | | 2001 | 6.59 | | 2000 | 5.71 | | 1999 | 5.64 | | 1998 | 5.79 | | 1997 | 7.91 | | 1996 | 7.46 | | 1995 | 8.11 | | 1994 | 11.11 | | 1993 | 6.7 | | 1992 | 11.46 | | 1991 | 11.94 | | 1990 | 10.15 | | 1989 | 5.36 | | 1988 | 4.4 |
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