Year | Value | 2018 | 15.1 | 2017 | 15 | 2016 | 14.5 | 2015 | 13.1 | 2014 | 12.4 | 2013 | 12 | 2012 | 11.7 | 2011 | 11.9 | 2010 | 12.6 | 2009 | 13.8 | 2008 | 15.4 | 2007 | 15.9 | 2006 | 16.3 | 2005 | 16.7 | 2004 | 17 | 2003 | 18.4 | 2001 | 18.6 | 2000 | 18.4 | 1999 | 19.5 | 1998 | 20.3 | 1997 | 22.4 | 1996 | 23.8 | 1995 | 25.5 | 1994 | 25.3 | 1993 | 25.7 | 1992 | 26.2 | 1991 | 27.9 | 1990 | 26.2 | 1989 | 28.3 | 1988 | 28 | 1987 | 28.8 | 1986 | 28.3 | 1985 | 28 |
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