| Year | Value | | 2021 | 6.211 | | 2020 | 6.065 | | 2019 | 5.728 | | 2018 | 5.472 | | 2017 | 5.265 | | 2016 | 5.073 | | 2015 | 5.008 | | 2014 | 4.844 | | 2013 | 4.743 | | 2012 | 4.676 | | 2011 | 4.612 | | 2010 | 4.587 | | 2009 | 4.66 | | 2008 | 4.657 | | 2007 | 4.676 | | 2006 | 4.648 | | 2005 | 4.694 | | 2004 | 4.678 | | 2003 | 4.667 | | 2002 | 5.128 | | 2001 | 4.707 | | 2000 | 4.953 | | 1999 | 5.073 | | 1998 | 5.252 | | 1997 | 5.41 | | 1996 | 5.626 | | 1995 | 5.75 | | 1994 | 5.812 | | 1993 | 5.686 | | 1992 | 5.638 | | 1991 | 5.715 | | 1990 | 5.794 | | 1989 | 5.925 | | 1988 | 6.035 | | 1987 | 6.281 | | 1986 | 6.354 | | 1985 | 6.473 | | 1984 | 6.594 | | 1983 | 6.798 | | 1982 | 6.998 | | 1981 | 7.159 | | 1980 | 7.294 | | 1979 | 7.501 | | 1978 | 7.567 | | 1977 | 7.775 | | 1976 | 7.962 | | 1975 | 8.168 | | 1974 | 8.368 | | 1973 | 8.598 | | 1972 | 8.833 | | 1971 | 9.025 | | 1970 | 9.304 | | 1969 | 9.536 | | 1968 | 9.82 | | 1967 | 10.044 | | 1966 | 10.376 | | 1965 | 10.583 | | 1964 | 10.961 | | 1963 | 11.235 | | 1962 | 11.505 | | 1961 | 11.938 | | 1960 | 12.386 |
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