| Year | Value | | 2021 | 4 | | 2020 | 3.3 | | 2019 | 3.9 | | 2018 | 3.3 | | 2017 | 3.4 | | 2016 | 3.1 | | 2015 | 2.9 | | 2014 | 2.9 | | 2013 | 3.2 | | 2012 | 3.3 | | 2011 | 3.2 | | 2010 | 3 | | 2009 | 3.1 | | 2008 | 3 | | 2007 | 3 | | 2006 | 3.4 | | 2005 | 3.8 | | 2004 | 4.6 | | 2003 | 3.5 | | 2002 | 2.8 | | 2001 | 3.2 | | 2000 | 3.4 | | 1999 | 3.3 | | 1998 | 3.1 | | 1997 | 3.4 | | 1996 | 3.6 | | 1995 | 3.4 | | 1994 | 4 | | 1993 | 4 | | 1992 | 4 | | 1991 | 4.1 | | 1990 | 4.1 | | 1989 | 4.5 | | 1988 | 4.6 | | 1987 | 4.6 | | 1986 | 6 | | 1985 | 5.6 |
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