| Year | Value | | 2017 | 4.49 | | 2016 | 4.67 | | 2015 | 4.59 | | 2014 | 4.59 | | 2013 | 4.78 | | 2012 | 4.71 | | 2011 | 4.38 | | 2010 | 4.52 | | 2009 | 4.83 | | 2008 | 7.39 | | 2007 | 7.85 | | 2006 | 7.57 | | 2005 | 7.45 | | 2004 | 7.46 | | 2003 | 7.43 | | 2002 | 7.48 | | 2001 | 7.57 | | 2000 | 5.48 | | 1999 | 5.5700001717 | | 1998 | 5.6100001335 | | 1997 | 5.6399998665 | | 1996 | 5.7800002098 | | 1995 | 5.4299998283 | | 1994 | 5.6210999489 | | 1993 | 5.8432002068 | | 1989 | 9.4411001205444 | | 1988 | 9.2573003768921 | | 1987 | 9.3353004455566 | | 1984 | 9.4638996124268 | | 1960 | 9.2583589553833 |
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