| Year | Value | | 2022 | 3.462 | | 2021 | 3.827 | | 2020 | 4.255 | | 2019 | 3.59 | | 2018 | 4.387 | | 2017 | 3.783 | | 2016 | 4.301 | | 2015 | 4.514 | | 2014 | 4.049 | | 2013 | 4.336 | | 2012 | 4.468 | | 2011 | 5.153 | | 2010 | 5.614 | | 2009 | 6.106 | | 2008 | 7.209 | | 2007 | 8.06 | | 2006 | 7.551 | | 2005 | 7.945 | | 2004 | 7.303 | | 2003 | 6.658 | | 2002 | 6.604 | | 2001 | 6.082 | | 2000 | 6.078 | | 1999 | 6.358 | | 1998 | 5.459 | | 1997 | 4.684 | | 1996 | 4.861 | | 1994 | 4.366 | | 1993 | 2.782 | | 1992 | 2.734 | | 1991 | 2.617 | | 1990 | 2.541 | | 1988 | 2.74 | | 1987 | 2.55 | | 1986 | 2.59 | | 1985 | 2.1 | | 1982 | 2.85 | | 1980 | 1.66 | | 1978 | 2.48 | | 1977 | 1.29 | | 1976 | 1.92 |
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