| Year | Value | | 2021 | 3.978 | | 2020 | 4.052 | | 2019 | 4.125 | | 2018 | 4.196 | | 2017 | 4.278 | | 2016 | 4.348 | | 2015 | 4.418 | | 2014 | 4.492 | | 2013 | 4.568 | | 2012 | 4.641 | | 2011 | 4.721 | | 2010 | 4.759 | | 2009 | 4.791 | | 2008 | 4.826 | | 2007 | 4.882 | | 2006 | 4.955 | | 2005 | 5.03 | | 2004 | 5.095 | | 2003 | 5.149 | | 2002 | 5.206 | | 2001 | 5.254 | | 2000 | 5.297 | | 1999 | 5.372 | | 1998 | 5.492 | | 1997 | 5.593 | | 1996 | 5.69 | | 1995 | 5.784 | | 1994 | 5.897 | | 1993 | 6.052 | | 1992 | 6.205 | | 1991 | 6.358 | | 1990 | 6.497 | | 1989 | 6.618 | | 1988 | 6.725 | | 1987 | 6.817 | | 1986 | 6.894 | | 1985 | 6.96 | | 1984 | 7.011 | | 1983 | 7.046 | | 1982 | 7.067 | | 1981 | 7.077 | | 1980 | 7.082 | | 1979 | 7.104 | | 1978 | 7.097 | | 1977 | 7.101 | | 1976 | 7.114 | | 1975 | 7.121 | | 1974 | 7.134 | | 1973 | 7.142 | | 1972 | 7.119 | | 1971 | 7.098 | | 1970 | 7.061 | | 1969 | 7.06 | | 1968 | 7.055 | | 1967 | 7.044 | | 1966 | 7.027 | | 1965 | 7.004 | | 1964 | 6.975 | | 1963 | 6.939 | | 1962 | 6.897 | | 1961 | 6.849 | | 1960 | 6.792 |
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