| Year | Value | | 2018 | 8.427 | | 2017 | 8.297 | | 2016 | 7.949 | | 2015 | 7.759 | | 2014 | 7.551 | | 2013 | 7.393 | | 2012 | 7.256 | | 2011 | 6.959 | | 2010 | 6.776 | | 2009 | 6.636 | | 2008 | 6.611 | | 2007 | 6.426 | | 2006 | 6.349 | | 2005 | 6.277 | | 2004 | 6.21 | | 2003 | 6.073 | | 2002 | 6.005 | | 2001 | 5.954 | | 2000 | 5.943 | | 1999 | 5.859 | | 1998 | 5.754 | | 1997 | 5.694 | | 1996 | 5.485 | | 1995 | 5.202 | | 1994 | 4.966 | | 1993 | 4.709 | | 1992 | 4.345 | | 1991 | 3.979 | | 1990 | 3.641 | | 1988 | 3 | | 1984 | 1.886 | | 1981 | 1.388 | | 1970 | 0.822 | | 1965 | 0.876 | | 1960 | 0.946 |
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