| Year | Value | | 2018 | 7.01 | | 2017 | 7.02 | | 2016 | 7 | | 2015 | 6.99 | | 2014 | 6.98 | | 2013 | 7.04 | | 2012 | 7 | | 2011 | 7.19 | | 2010 | 7.18 | | 2009 | 7.14 | | 2008 | 7.11 | | 2007 | 7.19 | | 2006 | 7.98 | | 2005 | 7.91 | | 2004 | 7.89 | | 2003 | 7.9 | | 2002 | 7.91 | | 2001 | 7.92 | | 2000 | 8.16 | | 1999 | 8.1000003815 | | 1998 | 8.1000003815 | | 1997 | 8 | | 1996 | 8.8000001907 | | 1995 | 8.8000001907 | | 1994 | 9.3000001907 |
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