| Year | Value | | 2020 | 5.365 | | 2019 | 4.67 | | 2018 | 4.318 | | 2017 | 4.046 | | 2016 | 4.243 | | 2015 | 4.519 | | 2014 | 4.157 | | 2013 | 4.186 | | 2012 | 3.88 | | 2011 | 4.118 | | 2010 | 4.784 | | 2009 | 5.85 | | 2008 | 5.22 | | 2007 | 5.97 | | 2006 | 6.5 | | 2005 | 7.67 | | 2004 | 8.38 | | 2003 | 8.22 | | 2002 | 8.76 | | 2001 | 7.9 | | 2000 | 7.74 | | 1999 | 8.88 | | 1998 | 9.17 | | 1997 | 10.6 | | 1996 | 11.35 | | 1995 | 12.27 | | 1994 | 13.03 | | 1993 | 13.82 | | 1992 | 14.22 | | 1991 | 14.661 | | 1990 | 15.9 |
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