| Year | Value | | 2021 | 3.983 | | 2020 | 4.038 | | 2019 | 4.104 | | 2018 | 4.157 | | 2017 | 4.218 | | 2016 | 4.293 | | 2015 | 4.356 | | 2014 | 4.418 | | 2013 | 4.418 | | 2012 | 4.412 | | 2011 | 4.406 | | 2010 | 4.404 | | 2009 | 4.393 | | 2008 | 4.411 | | 2007 | 4.442 | | 2006 | 4.477 | | 2005 | 4.475 | | 2004 | 4.518 | | 2003 | 4.572 | | 2002 | 4.626 | | 2001 | 4.692 | | 2000 | 4.756 | | 1999 | 4.821 | | 1998 | 4.903 | | 1997 | 4.987 | | 1996 | 5.118 | | 1995 | 5.243 | | 1994 | 5.314 | | 1993 | 5.375 | | 1992 | 5.445 | | 1991 | 5.545 | | 1990 | 5.66 | | 1989 | 5.775 | | 1988 | 5.883 | | 1987 | 5.99 | | 1986 | 6.114 | | 1985 | 6.217 | | 1984 | 6.311 | | 1983 | 6.416 | | 1982 | 6.516 | | 1981 | 6.613 | | 1980 | 6.706 | | 1979 | 6.783 | | 1978 | 6.866 | | 1977 | 6.945 | | 1976 | 7.016 | | 1975 | 7.073 | | 1974 | 7.126 | | 1973 | 7.091 | | 1972 | 7.056 | | 1971 | 7.028 | | 1970 | 7.001 | | 1969 | 6.97 | | 1968 | 6.917 | | 1967 | 6.94 | | 1966 | 6.941 | | 1965 | 6.951 | | 1964 | 6.964 | | 1963 | 6.965 | | 1962 | 6.97 | | 1961 | 6.97 | | 1960 | 6.97 |
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