| Year | Value | | 2021 | 3.304 | | 2020 | 3.333 | | 2019 | 3.368 | | 2018 | 3.41 | | 2017 | 3.431 | | 2016 | 3.471 | | 2015 | 3.53 | | 2014 | 3.599 | | 2013 | 3.689 | | 2012 | 3.774 | | 2011 | 3.857 | | 2010 | 3.876 | | 2009 | 3.889 | | 2008 | 3.905 | | 2007 | 3.879 | | 2006 | 3.837 | | 2005 | 3.799 | | 2004 | 3.781 | | 2003 | 3.744 | | 2002 | 3.847 | | 2001 | 3.958 | | 2000 | 4.065 | | 1999 | 4.158 | | 1998 | 4.253 | | 1997 | 4.325 | | 1996 | 4.39 | | 1995 | 4.385 | | 1994 | 4.418 | | 1993 | 4.457 | | 1992 | 4.527 | | 1991 | 4.594 | | 1990 | 4.637 | | 1989 | 4.7 | | 1988 | 4.754 | | 1987 | 4.822 | | 1986 | 4.889 | | 1985 | 4.935 | | 1984 | 4.941 | | 1983 | 4.954 | | 1982 | 4.982 | | 1981 | 4.988 | | 1980 | 4.99 | | 1979 | 4.996 | | 1978 | 4.996 | | 1977 | 4.99 | | 1976 | 4.987 | | 1975 | 5.029 | | 1974 | 5.079 | | 1973 | 5.144 | | 1972 | 5.257 | | 1971 | 5.397 | | 1970 | 5.587 | | 1969 | 5.785 | | 1968 | 5.971 | | 1967 | 6.146 | | 1966 | 6.308 | | 1965 | 6.363 | | 1964 | 6.411 | | 1963 | 6.486 | | 1962 | 6.557 | | 1961 | 6.625 | | 1960 | 6.553 |
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