| Year | Value | | 2021 | 4.726 | | 2020 | 4.795 | | 2019 | 4.864 | | 2018 | 4.923 | | 2017 | 4.993 | | 2016 | 5.033 | | 2015 | 5.088 | | 2014 | 5.122 | | 2013 | 5.122 | | 2012 | 5.098 | | 2011 | 5.207 | | 2010 | 5.293 | | 2009 | 5.357 | | 2008 | 5.425 | | 2007 | 5.485 | | 2006 | 5.563 | | 2005 | 5.61 | | 2004 | 5.607 | | 2003 | 5.605 | | 2002 | 5.623 | | 2001 | 5.665 | | 2000 | 5.686 | | 1999 | 5.734 | | 1998 | 5.789 | | 1997 | 5.838 | | 1996 | 5.848 | | 1995 | 5.865 | | 1994 | 5.927 | | 1993 | 6.011 | | 1992 | 6.078 | | 1991 | 6.135 | | 1990 | 6.195 | | 1989 | 6.289 | | 1988 | 6.371 | | 1987 | 6.399 | | 1986 | 6.475 | | 1985 | 6.569 | | 1984 | 6.669 | | 1983 | 6.749 | | 1982 | 6.796 | | 1981 | 6.851 | | 1980 | 6.906 | | 1979 | 6.954 | | 1978 | 6.979 | | 1977 | 6.987 | | 1976 | 6.989 | | 1975 | 7.001 | | 1974 | 6.998 | | 1973 | 6.962 | | 1972 | 6.951 | | 1971 | 6.944 | | 1970 | 6.909 | | 1969 | 6.878 | | 1968 | 6.835 | | 1967 | 6.798 | | 1966 | 6.791 | | 1965 | 6.777 | | 1964 | 6.775 | | 1963 | 6.77 | | 1962 | 6.758 | | 1961 | 6.754 | | 1960 | 6.725 |
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