| Year | Value | | 2019 | 3.111 | | 2018 | 3.2 | | 2017 | 3.164 | | 2016 | 3.227 | | 2015 | 3.291 | | 2014 | 3.362 | | 2013 | 3.373 | | 2012 | 3.409 | | 2011 | 3.359 | | 2010 | 3.582 | | 2009 | 3.596 | | 2008 | 3.612 | | 2007 | 3.648 | | 2006 | 3.511 | | 2005 | 3.533 | | 2004 | 3.512 | | 2003 | 3.508 | | 2002 | 3.521 | | 2001 | 3.512 | | 2000 | 3.545 | | 1999 | 3.516 | | 1998 | 3.541 | | 1997 | 3.609 | | 1996 | 3.673 | | 1995 | 3.703 | | 1994 | 3.684 | | 1993 | 3.724 | | 1992 | 3.687 | | 1991 | 3.732 | | 1990 | 3.701 | | 1989 | 3.99 | | 1988 | 3.964 | | 1987 | 3.906 | | 1986 | 3.877 | | 1985 | 3.799 | | 1980 | 3.362 |
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