| Year | Value | | 2021 | 3.851 | | 2020 | 3.918 | | 2019 | 3.978 | | 2018 | 4.043 | | 2017 | 4.109 | | 2016 | 4.163 | | 2015 | 4.183 | | 2014 | 4.247 | | 2013 | 4.358 | | 2012 | 4.466 | | 2011 | 4.607 | | 2010 | 4.755 | | 2009 | 4.87 | | 2008 | 4.933 | | 2007 | 4.997 | | 2006 | 5.056 | | 2005 | 5.095 | | 2004 | 5.133 | | 2003 | 5.172 | | 2002 | 5.225 | | 2001 | 5.323 | | 2000 | 5.403 | | 1999 | 5.549 | | 1998 | 5.675 | | 1997 | 5.801 | | 1996 | 5.888 | | 1995 | 5.946 | | 1994 | 5.99 | | 1993 | 6.041 | | 1992 | 6.08 | | 1991 | 6.122 | | 1990 | 6.161 | | 1989 | 6.177 | | 1988 | 6.214 | | 1987 | 6.287 | | 1986 | 6.307 | | 1985 | 6.353 | | 1984 | 6.41 | | 1983 | 6.477 | | 1982 | 6.554 | | 1981 | 6.637 | | 1980 | 6.725 | | 1979 | 6.813 | | 1978 | 6.898 | | 1977 | 6.976 | | 1976 | 7.044 | | 1975 | 7.102 | | 1974 | 7.149 | | 1973 | 7.188 | | 1972 | 7.221 | | 1971 | 7.248 | | 1970 | 7.27 | | 1969 | 7.286 | | 1968 | 7.297 | | 1967 | 7.304 | | 1966 | 7.307 | | 1965 | 7.308 | | 1964 | 7.307 | | 1963 | 7.305 | | 1962 | 7.303 | | 1961 | 7.301 | | 1960 | 7.3 |
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