| Year | Value | | 2021 | 4.111 | | 2020 | 4.047 | | 2019 | 3.983 | | 2018 | 3.86 | | 2017 | 3.864 | | 2016 | 3.877 | | 2015 | 3.851 | | 2014 | 3.768 | | 2013 | 3.715 | | 2012 | 3.771 | | 2011 | 3.719 | | 2010 | 3.678 | | 2009 | 3.526 | | 2008 | 3.577 | | 2007 | 3.62 | | 2006 | 3.64 | | 2005 | 3.644 | | 2004 | 3.694 | | 2003 | 3.692 | | 2002 | 3.701 | | 2001 | 3.654 | | 2000 | 3.64 | | 1999 | 3.654 | | 1998 | 3.672 | | 1997 | 3.653 | | 1996 | 3.639 | | 1995 | 3.796 | | 1994 | 3.859 | | 1993 | 3.936 | | 1992 | 4.018 | | 1991 | 4.179 | | 1990 | 4.425 | | 1989 | 4.639 | | 1988 | 4.879 | | 1987 | 5.079 | | 1986 | 5.327 | | 1985 | 5.562 | | 1984 | 5.799 | | 1983 | 6.024 | | 1982 | 6.238 | | 1981 | 6.397 | | 1980 | 6.272 | | 1979 | 6.152 | | 1978 | 6.02 | | 1977 | 5.924 | | 1976 | 5.834 | | 1975 | 5.759 | | 1974 | 5.682 | | 1973 | 5.613 | | 1972 | 5.596 | | 1971 | 5.567 | | 1970 | 5.732 | | 1969 | 5.922 | | 1968 | 6.166 | | 1967 | 6.449 | | 1966 | 6.737 | | 1965 | 7.081 | | 1964 | 7.424 | | 1963 | 7.788 | | 1962 | 8.177 | | 1961 | 8.613 | | 1960 | 8.721 |
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