| Year | Value | | 2021 | 4.469 | | 2020 | 4.543 | | 2019 | 4.619 | | 2018 | 4.696 | | 2017 | 4.775 | | 2016 | 4.855 | | 2015 | 4.937 | | 2014 | 5.022 | | 2013 | 5.108 | | 2012 | 5.197 | | 2011 | 5.286 | | 2010 | 5.376 | | 2009 | 5.465 | | 2008 | 5.552 | | 2007 | 5.638 | | 2006 | 5.82 | | 2005 | 6.171 | | 2004 | 6.517 | | 2003 | 6.851 | | 2002 | 7.191 | | 2001 | 7.369 | | 2000 | 7.513 | | 1999 | 7.672 | | 1998 | 7.817 | | 1997 | 7.965 | | 1996 | 7.985 | | 1995 | 7.962 | | 1994 | 7.989 | | 1993 | 8.005 | | 1992 | 7.999 | | 1991 | 8.004 | | 1990 | 7.988 | | 1989 | 7.993 | | 1988 | 8.012 | | 1987 | 7.947 | | 1986 | 7.907 | | 1985 | 7.769 | | 1984 | 7.437 | | 1983 | 7.134 | | 1982 | 6.813 | | 1981 | 6.829 | | 1980 | 6.85 | | 1979 | 6.874 | | 1978 | 6.897 | | 1977 | 6.913 | | 1976 | 6.922 | | 1975 | 6.924 | | 1974 | 6.92 | | 1973 | 6.912 | | 1972 | 6.903 | | 1971 | 6.892 | | 1970 | 6.881 | | 1969 | 6.869 | | 1968 | 6.855 | | 1967 | 6.838 | | 1966 | 6.819 | | 1965 | 6.8 | | 1964 | 6.781 | | 1963 | 6.763 | | 1962 | 6.747 | | 1961 | 6.733 | | 1960 | 6.721 |
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