| Year | Value | | 2017 | 4.53 | | 2016 | 5.16 | | 2015 | 4.87 | | 2014 | 4.44 | | 2013 | 5.33 | | 2012 | 4.96 | | 2011 | 5.64 | | 2010 | 5.57 | | 2009 | 5.68 | | 2008 | 5.91 | | 2007 | 5.92 | | 2006 | 6.02 | | 2005 | 5.98 | | 2004 | 6.51 | | 2003 | 6.7 | | 2002 | 6.3 | | 2001 | 7.18 | | 2000 | 7.25 | | 1999 | 9.3 | | 1998 | 7.12 | | 1997 | 8.19 | | 1996 | 8.02 | | 1995 | 10.24 | | 1994 | 19.31 | | 1993 | 19.46 | | 1992 | 15.67 | | 1991 | 19.23 | | 1990 | 13.85 | | 1989 | 16.21 | | 1988 | 13.11 |
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