| Year | Value | | 2021 | 12.4 | | 2020 | 12.6 | | 2019 | 9.8 | | 2018 | 9.9 | | 2017 | 9.9 | | 2016 | 10.8 | | 2015 | 11 | | 2014 | 12.5 | | 2013 | 12.5 | | 2012 | 13.6 | | 2011 | 14.5 | | 2010 | 14.8 | | 2009 | 14.6 | | 2008 | 14.2 | | 2007 | 13.8 | | 2006 | 13.4 | | 2005 | 15.7 | | 2004 | 15.2 | | 2003 | 14.3 | | 2002 | 13.7 | | 2001 | 15 | | 2000 | 15.4 | | 1999 | 15.5 | | 1998 | 14.5 | | 1997 | 15.9 | | 1996 | 16.3 | | 1995 | 14.9 | | 1994 | 17 | | 1993 | 17.6 | | 1992 | 18.1 | | 1991 | 18.2 | | 1990 | 18.6 | | 1989 | 17.5 | | 1988 | 16.1 | | 1987 | 16.1 | | 1986 | 16.9 | | 1985 | 17.6 |
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