| Year | Value | | 2021 | 4.772 | | 2020 | 4.869 | | 2019 | 4.971 | | 2018 | 5.069 | | 2017 | 5.162 | | 2016 | 5.324 | | 2015 | 5.478 | | 2014 | 5.595 | | 2013 | 5.697 | | 2012 | 5.793 | | 2011 | 5.869 | | 2010 | 5.936 | | 2009 | 5.992 | | 2008 | 6.05 | | 2007 | 6.11 | | 2006 | 6.17 | | 2005 | 6.182 | | 2004 | 6.216 | | 2003 | 6.28 | | 2002 | 6.339 | | 2001 | 6.431 | | 2000 | 6.519 | | 1999 | 6.586 | | 1998 | 6.661 | | 1997 | 6.724 | | 1996 | 6.779 | | 1995 | 6.814 | | 1994 | 6.843 | | 1993 | 6.889 | | 1992 | 6.936 | | 1991 | 6.973 | | 1990 | 7.009 | | 1989 | 7.047 | | 1988 | 7.109 | | 1987 | 7.168 | | 1986 | 7.218 | | 1985 | 7.242 | | 1984 | 7.245 | | 1983 | 7.251 | | 1982 | 7.253 | | 1981 | 7.256 | | 1980 | 7.222 | | 1979 | 7.194 | | 1978 | 7.148 | | 1977 | 7.046 | | 1976 | 6.952 | | 1975 | 6.913 | | 1974 | 6.842 | | 1973 | 6.77 | | 1972 | 6.719 | | 1971 | 6.698 | | 1970 | 6.685 | | 1969 | 6.664 | | 1968 | 6.645 | | 1967 | 6.585 | | 1966 | 6.527 | | 1965 | 6.471 | | 1964 | 6.419 | | 1963 | 6.356 | | 1962 | 6.322 | | 1961 | 6.286 | | 1960 | 6.248 |
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