| Year | Value | | 2021 | 4.418 | | 2020 | 4.472 | | 2019 | 4.538 | | 2018 | 4.611 | | 2017 | 4.742 | | 2016 | 4.759 | | 2015 | 4.813 | | 2014 | 4.867 | | 2013 | 4.92 | | 2012 | 4.974 | | 2011 | 5.029 | | 2010 | 5.088 | | 2009 | 5.152 | | 2008 | 5.221 | | 2007 | 5.296 | | 2006 | 5.376 | | 2005 | 5.46 | | 2004 | 5.545 | | 2003 | 5.642 | | 2002 | 5.705 | | 2001 | 5.756 | | 2000 | 5.814 | | 1999 | 5.847 | | 1998 | 5.896 | | 1997 | 5.951 | | 1996 | 6.005 | | 1995 | 6.089 | | 1994 | 6.161 | | 1993 | 6.306 | | 1992 | 6.456 | | 1991 | 6.606 | | 1990 | 6.73 | | 1989 | 6.844 | | 1988 | 6.963 | | 1987 | 7.078 | | 1986 | 7.184 | | 1985 | 7.265 | | 1984 | 7.349 | | 1983 | 7.423 | | 1982 | 7.487 | | 1981 | 7.541 | | 1980 | 7.587 | | 1979 | 7.628 | | 1978 | 7.669 | | 1977 | 7.828 | | 1976 | 7.877 | | 1975 | 7.91 | | 1974 | 7.929 | | 1973 | 7.939 | | 1972 | 7.942 | | 1971 | 7.941 | | 1970 | 7.936 | | 1969 | 7.927 | | 1968 | 7.912 | | 1967 | 7.893 | | 1966 | 7.868 | | 1965 | 7.841 | | 1964 | 7.811 | | 1963 | 7.781 | | 1962 | 7.75 | | 1961 | 7.72 | | 1960 | 7.691 |
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