| Year | Value | | 2021 | 4.859 | | 2020 | 5.703 | | 2019 | 4.626 | | 2018 | 4.605 | | 2017 | 4.619 | | 2016 | 4.633 | | 2015 | 4.635 | | 2014 | 4.597 | | 2013 | 4.535 | | 2012 | 4.47 | | 2011 | 4.444 | | 2010 | 4.491 | | 2009 | 4.598 | | 2008 | 4.808 | | 2007 | 5.105 | | 2006 | 5.357 | | 2005 | 5.614 | | 2004 | 5.748 | | 2003 | 5.758 | | 2002 | 5.699 | | 2001 | 5.615 | | 2000 | 5.547 | | 1999 | 5.539 | | 1998 | 6.29 | | 1997 | 5.703 | | 1996 | 5.866 | | 1995 | 6.051 | | 1994 | 6.248 | | 1993 | 6.488 | | 1992 | 6.704 | | 1991 | 6.896 | | 1990 | 7.288 | | 1989 | 7.618 | | 1988 | 8.563 | | 1987 | 9.736 | | 1986 | 9.807 | | 1985 | 10.474 | | 1984 | 10.763 | | 1983 | 10.612 | | 1982 | 10.009 | | 1981 | 10.292 | | 1980 | 10.535 | | 1979 | 13.976 | | 1978 | 12.768 | | 1977 | 11.584 | | 1976 | 11.666 | | 1975 | 11.866 | | 1974 | 12.029 | | 1973 | 12.164 | | 1972 | 16.285 | | 1971 | 12.524 | | 1970 | 12.764 | | 1969 | 13.135 | | 1968 | 13.637 | | 1967 | 14.262 | | 1966 | 14.914 | | 1965 | 15.64 | | 1964 | 16.35 | | 1963 | 17.006 | | 1962 | 17.602 | | 1961 | 18.161 | | 1960 | 18.906 |
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