| Year | Value | | 2021 | 3.735 | | 2020 | 3.778 | | 2019 | 3.813 | | 2018 | 3.855 | | 2017 | 3.91 | | 2016 | 3.963 | | 2015 | 4.006 | | 2014 | 4.06 | | 2013 | 4.093 | | 2012 | 4.124 | | 2011 | 4.151 | | 2010 | 4.15 | | 2009 | 4.148 | | 2008 | 4.138 | | 2007 | 4.124 | | 2006 | 4.121 | | 2005 | 4.158 | | 2004 | 4.226 | | 2003 | 4.294 | | 2002 | 4.364 | | 2001 | 4.444 | | 2000 | 4.48 | | 1999 | 4.516 | | 1998 | 4.575 | | 1997 | 4.632 | | 1996 | 4.688 | | 1995 | 4.765 | | 1994 | 4.824 | | 1993 | 4.894 | | 1992 | 4.95 | | 1991 | 4.992 | | 1990 | 5.029 | | 1989 | 5.076 | | 1988 | 5.158 | | 1987 | 5.245 | | 1986 | 5.318 | | 1985 | 5.381 | | 1984 | 5.449 | | 1983 | 5.527 | | 1982 | 5.593 | | 1981 | 5.668 | | 1980 | 5.737 | | 1979 | 5.809 | | 1978 | 5.869 | | 1977 | 5.936 | | 1976 | 6.016 | | 1975 | 6.089 | | 1974 | 6.164 | | 1973 | 6.244 | | 1972 | 6.315 | | 1971 | 6.379 | | 1970 | 6.433 | | 1969 | 6.493 | | 1968 | 6.54 | | 1967 | 6.592 | | 1966 | 6.613 | | 1965 | 6.642 | | 1964 | 6.683 | | 1963 | 6.729 | | 1962 | 6.777 | | 1961 | 6.818 | | 1960 | 6.863 |
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