| Year | Value | | 2021 | 2.747 | | 2020 | 2.798 | | 2019 | 2.835 | | 2018 | 2.886 | | 2017 | 2.939 | | 2016 | 2.994 | | 2015 | 3.049 | | 2014 | 3.101 | | 2013 | 3.17 | | 2012 | 3.215 | | 2011 | 3.283 | | 2010 | 3.404 | | 2009 | 3.514 | | 2008 | 3.613 | | 2007 | 3.696 | | 2006 | 3.762 | | 2005 | 3.814 | | 2004 | 3.857 | | 2003 | 3.899 | | 2002 | 3.948 | | 2001 | 4.007 | | 2000 | 4.001 | | 1999 | 4.082 | | 1998 | 4.177 | | 1997 | 4.283 | | 1996 | 4.428 | | 1995 | 4.574 | | 1994 | 4.715 | | 1993 | 4.834 | | 1992 | 4.945 | | 1991 | 5.124 | | 1990 | 5.384 | | 1989 | 5.628 | | 1988 | 5.886 | | 1987 | 6.134 | | 1986 | 6.326 | | 1985 | 6.476 | | 1984 | 6.606 | | 1983 | 6.741 | | 1982 | 6.875 | | 1981 | 7.011 | | 1980 | 7.155 | | 1979 | 7.269 | | 1978 | 7.347 | | 1977 | 7.41 | | 1976 | 7.435 | | 1975 | 7.471 | | 1974 | 7.509 | | 1973 | 7.56 | | 1972 | 7.612 | | 1971 | 7.653 | | 1970 | 7.694 | | 1969 | 7.668 | | 1968 | 7.601 | | 1967 | 7.583 | | 1966 | 7.554 | | 1965 | 7.514 | | 1964 | 7.469 | | 1963 | 7.441 | | 1962 | 7.441 | | 1961 | 7.431 | | 1960 | 7.485 |
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