| Year | Value | | 2021 | 5.469 | | 2020 | 4.16 | | 2019 | 4.119 | | 2018 | 4.074 | | 2017 | 4.0676 | | 2016 | 4.0194 | | 2015 | 3.9516 | | 2014 | 3.9345 | | 2013 | 3.688 | | 2012 | 3.675 | | 2011 | 3.637 | | 2010 | 3.599 | | 2009 | 3.584 | | 2008 | 3.557 | | 2007 | 3.561 | | 2006 | 3.551 | | 2005 | 3.539 | | 2004 | 3.499 | | 2003 | 3.509 | | 2002 | 3.492 | | 2001 | 3.443 | | 2000 | 3.381 | | 1999 | 3.088 | | 1998 | 3.039 | | 1997 | 3.12 | | 1996 | 2.99 | | 1995 | 3.001 | | 1994 | 2.936 | | 1993 | 2.868 | | 1992 | 2.681 | | 1991 | 2.676 | | 1990 | 2.722 | | 1979 | 2.2 | | 1978 | 2.1 | | 1977 | 2.1 | | 1976 | 2.1 | | 1975 | 2 | | 1974 | 2 | | 1973 | 1.9 | | 1972 | 1.9 | | 1971 | 1.9 | | 1970 | 1.8 |
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