| Year | Value | | 2021 | 3.47 | | 2020 | 3.555 | | 2019 | 3.62 | | 2018 | 3.685 | | 2017 | 3.759 | | 2016 | 3.827 | | 2015 | 3.904 | | 2014 | 4.005 | | 2013 | 4.106 | | 2012 | 4.169 | | 2011 | 4.234 | | 2010 | 4.297 | | 2009 | 4.36 | | 2008 | 4.432 | | 2007 | 4.51 | | 2006 | 4.526 | | 2005 | 4.637 | | 2004 | 4.754 | | 2003 | 4.88 | | 2002 | 5.008 | | 2001 | 5.123 | | 2000 | 5.257 | | 1999 | 5.393 | | 1998 | 5.508 | | 1997 | 5.642 | | 1996 | 5.771 | | 1995 | 5.894 | | 1994 | 6.008 | | 1993 | 6.112 | | 1992 | 6.207 | | 1991 | 6.291 | | 1990 | 6.364 | | 1989 | 6.425 | | 1988 | 6.476 | | 1987 | 6.518 | | 1986 | 6.554 | | 1985 | 6.585 | | 1984 | 6.615 | | 1983 | 6.644 | | 1982 | 6.674 | | 1981 | 6.704 | | 1980 | 6.734 | | 1979 | 6.761 | | 1978 | 6.783 | | 1977 | 6.799 | | 1976 | 6.808 | | 1975 | 6.812 | | 1974 | 6.811 | | 1973 | 6.808 | | 1972 | 6.805 | | 1971 | 6.802 | | 1970 | 6.801 | | 1969 | 6.8 | | 1968 | 6.8 | | 1967 | 6.8 | | 1966 | 6.8 | | 1965 | 6.8 | | 1964 | 6.8 | | 1963 | 6.8 | | 1962 | 6.8 | | 1961 | 6.8 | | 1960 | 6.8 |
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