| Year | Value | | 2020 | 3.345 | | 2019 | 3.263 | | 2018 | 3.289 | | 2017 | 3.184 | | 2016 | 3.19 | | 2015 | 3.175 | | 2014 | 3.189 | | 2013 | 3.162 | | 2012 | 3.115 | | 2011 | 3.108 | | 2010 | 3.101 | | 2009 | 3.176 | | 2008 | 3.274 | | 2007 | 3.178 | | 2006 | 3.049 | | 2005 | 2.968 | | 2004 | 2.917 | | 2003 | 2.824 | | 2002 | 2.77 | | 2001 | 2.662 | | 2000 | 2.85 | | 1999 | 2.677 | | 1998 | 2.681 | | 1997 | 2.791 | | 1996 | 2.749 | | 1995 | 2.734 | | 1994 | 2.797 | | 1993 | 2.822 | | 1992 | 3.076 | | 1991 | 3.573 | | 1990 | 3.51 | | 1989 | 4.162 | | 1988 | 4.183 | | 1987 | 4.112 | | 1986 | 4.077 | | 1985 | 4.099 | | 1980 | 3.623 |
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