| Year | Value | | 2021 | 5.211 | | 2020 | 4.975 | | 2019 | 4.834 | | 2018 | 4.7 | | 2017 | 4.556 | | 2016 | 4.42 | | 2015 | 4.29 | | 2014 | 4.162 | | 2013 | 4.043 | | 2012 | 3.926 | | 2011 | 3.813 | | 2010 | 3.751 | | 2009 | 3.684 | | 2008 | 3.615 | | 2007 | 3.551 | | 2006 | 3.491 | | 2005 | 3.438 | | 2004 | 3.396 | | 2003 | 3.36 | | 2002 | 3.339 | | 2001 | 3.326 | | 2000 | 3.331 | | 1999 | 3.346 | | 1998 | 3.375 | | 1997 | 3.415 | | 1996 | 3.466 | | 1995 | 3.532 | | 1994 | 3.603 | | 1993 | 3.677 | | 1992 | 3.756 | | 1991 | 3.817 | | 1990 | 3.87 | | 1989 | 3.926 | | 1988 | 3.981 | | 1987 | 4.041 | | 1986 | 4.089 | | 1985 | 4.168 | | 1984 | 4.235 | | 1983 | 4.319 | | 1982 | 4.405 | | 1981 | 4.492 | | 1980 | 4.573 | | 1979 | 4.687 | | 1978 | 4.809 | | 1977 | 4.952 | | 1976 | 5.13 | | 1975 | 5.339 | | 1974 | 5.595 | | 1973 | 5.868 | | 1972 | 6.18 | | 1971 | 6.487 | | 1970 | 6.75 | | 1969 | 7.001 | | 1968 | 7.257 | | 1967 | 7.517 | | 1966 | 7.827 | | 1965 | 8.278 | | 1964 | 8.7 | | 1963 | 9.268 | | 1962 | 10.449 | | 1961 | 10.73 | | 1960 | 11.655 |
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