| Year | Value | | 2021 | 4.457 | | 2020 | 4.542 | | 2019 | 4.617 | | 2018 | 4.681 | | 2017 | 4.761 | | 2016 | 4.829 | | 2015 | 4.898 | | 2014 | 4.912 | | 2013 | 4.903 | | 2012 | 4.895 | | 2011 | 4.96 | | 2010 | 5.017 | | 2009 | 5.05 | | 2008 | 5.074 | | 2007 | 5.065 | | 2006 | 5.044 | | 2005 | 5.039 | | 2004 | 5.094 | | 2003 | 5.172 | | 2002 | 5.248 | | 2001 | 5.327 | | 2000 | 5.38 | | 1999 | 5.447 | | 1998 | 5.504 | | 1997 | 5.596 | | 1996 | 5.707 | | 1995 | 5.788 | | 1994 | 5.874 | | 1993 | 5.961 | | 1992 | 6.053 | | 1991 | 6.116 | | 1990 | 6.168 | | 1989 | 6.231 | | 1988 | 6.282 | | 1987 | 6.355 | | 1986 | 6.425 | | 1985 | 6.487 | | 1984 | 6.547 | | 1983 | 6.597 | | 1982 | 6.64 | | 1981 | 6.679 | | 1980 | 6.728 | | 1979 | 6.779 | | 1978 | 6.837 | | 1977 | 6.871 | | 1976 | 6.903 | | 1975 | 6.933 | | 1974 | 6.954 | | 1973 | 6.96 | | 1972 | 7.012 | | 1971 | 7.059 | | 1970 | 7.027 | | 1969 | 7.002 | | 1968 | 6.971 | | 1967 | 6.941 | | 1966 | 6.902 | | 1965 | 6.849 | | 1964 | 6.803 | | 1963 | 6.75 | | 1962 | 6.692 | | 1961 | 6.628 | | 1960 | 6.647 |
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