| Year | Value | | 2021 | 7.22 | | 2020 | 7.505 | | 2019 | 5.56 | | 2018 | 5.505 | | 2017 | 5.262 | | 2016 | 4.973 | | 2015 | 4.841 | | 2014 | 4.645 | | 2013 | 4.593 | | 2012 | 4.56 | | 2011 | 4.661 | | 2010 | 4.659 | | 2009 | 4.568 | | 2008 | 4.596 | | 2007 | 4.864 | | 2006 | 5.172 | | 2005 | 5.037 | | 2004 | 5.032 | | 2003 | 5.021 | | 2002 | 5.014 | | 2001 | 4.992 | | 2000 | 4.986 | | 1999 | 6.437 | | 1998 | 5.569 | | 1997 | 4.838 | | 1996 | 4.878 | | 1995 | 4.919 | | 1994 | 4.968 | | 1993 | 5.042 | | 1992 | 5.157 | | 1991 | 5.327 | | 1990 | 5.421 | | 1989 | 5.475 | | 1988 | 5.58 | | 1987 | 5.658 | | 1986 | 5.856 | | 1985 | 6.002 | | 1984 | 6.157 | | 1983 | 6.301 | | 1982 | 6.422 | | 1981 | 6.494 | | 1980 | 6.676 | | 1979 | 6.712 | | 1978 | 6.915 | | 1977 | 7.135 | | 1976 | 7.343 | | 1975 | 7.506 | | 1974 | 7.705 | | 1973 | 7.871 | | 1972 | 8.097 | | 1971 | 8.258 | | 1970 | 8.393 | | 1969 | 8.628 | | 1968 | 8.776 | | 1967 | 8.933 | | 1966 | 9.131 | | 1965 | 9.379 | | 1964 | 9.652 | | 1963 | 9.903 | | 1962 | 10.134 | | 1961 | 10.362 | | 1960 | 10.631 |
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