| Year | Value | | 2021 | 3.516 | | 2020 | 3.448 | | 2019 | 3.39 | | 2018 | 3.353 | | 2017 | 3.299 | | 2016 | 3.147 | | 2015 | 3.046 | | 2014 | 2.894 | | 2013 | 2.824 | | 2012 | 2.714 | | 2011 | 2.655 | | 2010 | 2.626 | | 2009 | 2.581 | | 2008 | 2.464 | | 2007 | 2.299 | | 2006 | 2.274 | | 2005 | 2.107 | | 2004 | 2.192 | | 2003 | 2.179 | | 2002 | 2.126 | | 2001 | 2.183 | | 2000 | 2.235 | | 1999 | 2.248 | | 1998 | 2.229 | | 1997 | 2.178 | | 1996 | 2.048 | | 1995 | 2.05 | | 1994 | 1.983 | | 1993 | 1.923 | | 1992 | 1.903 | | 1991 | 1.889 | | 1990 | 1.866 | | 1989 | 1.9 | | 1988 | 1.9 | | 1987 | 1.8 | | 1986 | 1.8 | | 1985 | 1.7 | | 1984 | 1.7 | | 1983 | 1.7 | | 1982 | 1.6 | | 1981 | 1.6 | | 1980 | 1.6 | | 1979 | 1.5 | | 1978 | 1.4 | | 1977 | 1.4 | | 1976 | 1.3 | | 1973 | 1.2 | | 1971 | 1.1 | | 1966 | 1.1 | | 1961 | 1.1 |
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