| Year | Value | | 2021 | 3.496 | | 2020 | 3.551 | | 2019 | 3.604 | | 2018 | 3.659 | | 2017 | 3.73 | | 2016 | 3.9 | | 2015 | 4.09 | | 2014 | 4.283 | | 2013 | 4.378 | | 2012 | 4.477 | | 2011 | 4.536 | | 2010 | 4.429 | | 2009 | 4.362 | | 2008 | 4.342 | | 2007 | 4.371 | | 2006 | 4.436 | | 2005 | 4.475 | | 2004 | 4.575 | | 2003 | 4.615 | | 2002 | 4.706 | | 2001 | 4.837 | | 2000 | 4.948 | | 1999 | 5.056 | | 1998 | 5.154 | | 1997 | 5.244 | | 1996 | 5.285 | | 1995 | 5.398 | | 1994 | 5.528 | | 1993 | 5.64 | | 1992 | 5.72 | | 1991 | 5.81 | | 1990 | 5.884 | | 1989 | 5.944 | | 1988 | 6.018 | | 1987 | 6.099 | | 1986 | 6.176 | | 1985 | 6.248 | | 1984 | 6.326 | | 1983 | 6.389 | | 1982 | 6.45 | | 1981 | 6.514 | | 1980 | 6.567 | | 1979 | 6.629 | | 1978 | 6.693 | | 1977 | 6.754 | | 1976 | 6.814 | | 1975 | 6.875 | | 1974 | 6.928 | | 1973 | 6.963 | | 1972 | 7.008 | | 1971 | 7.056 | | 1970 | 7.079 | | 1969 | 7.099 | | 1968 | 7.046 | | 1967 | 6.997 | | 1966 | 6.97 | | 1965 | 6.945 | | 1964 | 6.928 | | 1963 | 6.901 | | 1962 | 6.869 | | 1961 | 5.8 | | 1960 | 5.3 |
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