| Year | Value | | 2021 | 4.171 | | 2020 | 4.234 | | 2019 | 4.296 | | 2018 | 4.367 | | 2017 | 4.433 | | 2016 | 4.487 | | 2015 | 4.564 | | 2014 | 4.626 | | 2013 | 4.691 | | 2012 | 4.75 | | 2011 | 4.791 | | 2010 | 4.822 | | 2009 | 4.792 | | 2008 | 4.733 | | 2007 | 4.666 | | 2006 | 4.656 | | 2005 | 4.663 | | 2004 | 4.687 | | 2003 | 4.7 | | 2002 | 4.719 | | 2001 | 4.712 | | 2000 | 4.764 | | 1999 | 4.853 | | 1998 | 4.926 | | 1997 | 4.962 | | 1996 | 4.966 | | 1995 | 4.973 | | 1994 | 5 | | 1993 | 5.057 | | 1992 | 5.111 | | 1991 | 5.16 | | 1990 | 5.214 | | 1989 | 5.305 | | 1988 | 5.395 | | 1987 | 5.487 | | 1986 | 5.571 | | 1985 | 5.645 | | 1984 | 5.725 | | 1983 | 5.811 | | 1982 | 5.898 | | 1981 | 5.984 | | 1980 | 6.065 | | 1979 | 6.143 | | 1978 | 6.21 | | 1977 | 6.271 | | 1976 | 6.317 | | 1975 | 6.356 | | 1974 | 6.39 | | 1973 | 6.42 | | 1972 | 6.444 | | 1971 | 6.416 | | 1970 | 6.376 | | 1969 | 6.35 | | 1968 | 6.327 | | 1967 | 6.29 | | 1966 | 6.267 | | 1965 | 6.229 | | 1964 | 6.183 | | 1963 | 6.147 | | 1962 | 6.099 | | 1961 | 6.093 | | 1960 | 6.085 |
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