| Year | Value | | 2021 | 6.067 | | 2020 | 5.747 | | 2019 | 5.12 | | 2018 | 5.013 | | 2017 | 4.947 | | 2016 | 4.904 | | 2015 | 4.875 | | 2014 | 4.856 | | 2013 | 4.835 | | 2012 | 4.818 | | 2011 | 4.789 | | 2010 | 4.755 | | 2009 | 4.701 | | 2008 | 4.651 | | 2007 | 4.586 | | 2006 | 4.547 | | 2005 | 4.526 | | 2004 | 4.541 | | 2003 | 4.569 | | 2002 | 4.626 | | 2001 | 4.694 | | 2000 | 4.768 | | 1999 | 4.836 | | 1998 | 4.896 | | 1997 | 4.948 | | 1996 | 5.001 | | 1995 | 5.057 | | 1994 | 5.118 | | 1993 | 5.187 | | 1992 | 5.278 | | 1991 | 5.347 | | 1990 | 5.374 | | 1989 | 5.783 | | 1988 | 5.42 | | 1987 | 5.408 | | 1986 | 5.416 | | 1985 | 5.439 | | 1984 | 5.491 | | 1983 | 5.566 | | 1982 | 5.652 | | 1981 | 5.747 | | 1980 | 5.839 | | 1979 | 5.918 | | 1978 | 5.997 | | 1977 | 6.063 | | 1976 | 6.164 | | 1975 | 6.305 | | 1974 | 6.498 | | 1973 | 6.75 | | 1972 | 7.033 | | 1971 | 7.331 | | 1970 | 7.654 | | 1969 | 7.864 | | 1968 | 8.055 | | 1967 | 8.213 | | 1966 | 8.379 | | 1965 | 8.525 | | 1964 | 8.728 | | 1963 | 8.972 | | 1962 | 9.253 | | 1961 | 9.576 | | 1960 | 9.941 |
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