| Year | Value | | 2021 | 2.839 | | 2020 | 2.89 | | 2019 | 2.921 | | 2018 | 2.962 | | 2017 | 3.015 | | 2016 | 3.068 | | 2015 | 3.117 | | 2014 | 3.174 | | 2013 | 3.226 | | 2012 | 3.255 | | 2011 | 3.299 | | 2010 | 3.356 | | 2009 | 3.473 | | 2008 | 3.536 | | 2007 | 3.564 | | 2006 | 3.617 | | 2005 | 3.682 | | 2004 | 3.731 | | 2003 | 3.757 | | 2002 | 3.805 | | 2001 | 3.9 | | 2000 | 3.996 | | 1999 | 4.066 | | 1998 | 4.098 | | 1997 | 4.207 | | 1996 | 4.347 | | 1995 | 4.487 | | 1994 | 4.623 | | 1993 | 4.774 | | 1992 | 4.949 | | 1991 | 5.097 | | 1990 | 5.25 | | 1989 | 5.414 | | 1988 | 5.612 | | 1987 | 5.816 | | 1986 | 5.998 | | 1985 | 6.17 | | 1984 | 6.253 | | 1983 | 6.334 | | 1982 | 6.404 | | 1981 | 6.472 | | 1980 | 6.543 | | 1979 | 6.584 | | 1978 | 6.643 | | 1977 | 6.695 | | 1976 | 6.743 | | 1975 | 6.75 | | 1974 | 6.752 | | 1973 | 6.682 | | 1972 | 6.719 | | 1971 | 6.762 | | 1970 | 6.757 | | 1969 | 6.758 | | 1968 | 6.757 | | 1967 | 6.751 | | 1966 | 6.815 | | 1965 | 6.821 | | 1964 | 6.819 | | 1963 | 6.855 | | 1962 | 6.794 | | 1961 | 6.774 | | 1960 | 6.752 |
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