| Year | Value | | 2021 | 3.335 | | 2020 | 3.397 | | 2019 | 3.469 | | 2018 | 3.58 | | 2017 | 3.643 | | 2016 | 3.724 | | 2015 | 3.8 | | 2014 | 3.844 | | 2013 | 3.952 | | 2012 | 4.088 | | 2011 | 4.219 | | 2010 | 4.367 | | 2009 | 4.514 | | 2008 | 4.646 | | 2007 | 4.719 | | 2006 | 4.748 | | 2005 | 4.784 | | 2004 | 4.83 | | 2003 | 4.907 | | 2002 | 5.016 | | 2001 | 5.091 | | 2000 | 5.137 | | 1999 | 5.181 | | 1998 | 5.246 | | 1997 | 5.26 | | 1996 | 5.306 | | 1995 | 5.366 | | 1994 | 5.446 | | 1993 | 5.574 | | 1992 | 5.746 | | 1991 | 5.935 | | 1990 | 6.129 | | 1989 | 6.323 | | 1988 | 6.505 | | 1987 | 6.682 | | 1986 | 6.848 | | 1985 | 6.975 | | 1984 | 7.123 | | 1983 | 7.263 | | 1982 | 7.402 | | 1981 | 7.513 | | 1980 | 7.598 | | 1979 | 7.679 | | 1978 | 7.746 | | 1977 | 7.803 | | 1976 | 7.843 | | 1975 | 7.877 | | 1974 | 7.907 | | 1973 | 7.939 | | 1972 | 7.975 | | 1971 | 8.003 | | 1970 | 8.024 | | 1969 | 8.031 | | 1968 | 8.038 | | 1967 | 8.046 | | 1966 | 8.056 | | 1965 | 8.028 | | 1964 | 7.92 | | 1963 | 7.864 | | 1962 | 7.803 | | 1961 | 7.718 | | 1960 | 7.632 |
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