| Year | Value | | 2021 | 7.192 | | 2020 | 5.627 | | 2019 | 5.468 | | 2018 | 5.276 | | 2017 | 5.18 | | 2016 | 4.986 | | 2015 | 5.034 | | 2014 | 5.059 | | 2013 | 4.62 | | 2012 | 4.541 | | 2011 | 4.379 | | 2010 | 4.552 | | 2009 | 4.433 | | 2008 | 4.421 | | 2007 | 4.304 | | 2006 | 4.176 | | 2005 | 4.078 | | 2004 | 4.047 | | 2003 | 4.055 | | 2002 | 3.999 | | 2001 | 4.089 | | 2000 | 3.995 | | 1999 | 4.088 | | 1998 | 4.114 | | 1997 | 3.985 | | 1996 | 4.019 | | 1995 | 4.057 | | 1994 | 3.961 | | 1993 | 3.894 | | 1992 | 3.865 | | 1991 | 3.823 | | 1990 | 3.758 | | 1989 | 3.826 | | 1988 | 3.83 | | 1987 | 3.902 | | 1986 | 3.935 | | 1985 | 4.014 | | 1984 | 3.975 | | 1983 | 3.927 | | 1982 | 4.005 | | 1981 | 4.219 | | 1980 | 4.462 | | 1979 | 4.577 | | 1978 | 4.587 | | 1977 | 4.849 | | 1976 | 5.134 | | 1975 | 5.402 | | 1974 | 5.706 | | 1973 | 5.874 | | 1972 | 6.335 | | 1971 | 6.529 | | 1970 | 6.943 | | 1969 | 7.15 | | 1968 | 7.477 | | 1967 | 7.829 | | 1966 | 8.192 | | 1965 | 8.626 | | 1964 | 9.019 | | 1963 | 9.36 | | 1962 | 9.716 | | 1961 | 9.926 | | 1960 | 10.232 |
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