| Year | Value | | 2018 | 12.98 | | 2017 | 13.05 | | 2016 | 13.11 | | 2015 | 13.17 | | 2014 | 13.21 | | 2013 | 13.3 | | 2012 | 13.35 | | 2011 | 13.4 | | 2010 | 13.51 | | 2009 | 13.62 | | 2008 | 13.71 | | 2007 | 13.87 | | 2006 | 13.97 | | 2005 | 14.08 | | 2004 | 14.2 | | 2003 | 14.26 | | 2002 | 14.43 | | 2001 | 14.58 | | 2000 | 14.69 | | 1999 | 14.8000001907 | | 1998 | 15 | | 1997 | 15.1000003815 | | 1996 | 15.1999998093 | | 1995 | 15.3999996185 | | 1994 | 15.5 | | 1993 | 15.6000003815 | | 1985 | 14.699999809265 | | 1980 | 13.699999809265 | | 1970 | 12.5 | | 1960 | 9 |
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