| Year | Value | | 2021 | 3.654 | | 2020 | 3.584 | | 2019 | 3.542 | | 2018 | 3.499 | | 2017 | 3.452 | | 2016 | 3.443 | | 2015 | 3.451 | | 2014 | 3.457 | | 2013 | 3.483 | | 2012 | 3.51 | | 2011 | 3.525 | | 2010 | 3.519 | | 2009 | 3.529 | | 2008 | 3.524 | | 2007 | 3.563 | | 2006 | 3.599 | | 2005 | 3.634 | | 2004 | 3.68 | | 2003 | 3.715 | | 2002 | 3.716 | | 2001 | 3.706 | | 2000 | 3.693 | | 1999 | 3.698 | | 1998 | 3.76 | | 1997 | 3.68 | | 1996 | 3.611 | | 1995 | 3.621 | | 1994 | 3.616 | | 1993 | 3.582 | | 1992 | 3.567 | | 1991 | 3.189 | | 1990 | 2.992 | | 1979 | 2.694 | | 1973 | 3.304 | | 1972 | 3.156 | | 1971 | 2.974 | | 1970 | 2.881 | | 1965 | 2.5 | | 1960 | 2.472 |
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