| Year | Value | | 2021 | 3.237 | | 2020 | 3.267 | | 2019 | 3.315 | | 2018 | 3.349 | | 2017 | 3.407 | | 2016 | 3.483 | | 2015 | 3.562 | | 2014 | 3.635 | | 2013 | 3.707 | | 2012 | 3.781 | | 2011 | 3.842 | | 2010 | 3.889 | | 2009 | 3.955 | | 2008 | 4.027 | | 2007 | 4.093 | | 2006 | 4.141 | | 2005 | 4.179 | | 2004 | 4.206 | | 2003 | 4.194 | | 2002 | 4.137 | | 2001 | 4.072 | | 2000 | 4.108 | | 1999 | 4.203 | | 1998 | 4.328 | | 1997 | 4.369 | | 1996 | 4.433 | | 1995 | 4.447 | | 1994 | 4.501 | | 1993 | 4.547 | | 1992 | 4.583 | | 1991 | 4.612 | | 1990 | 4.644 | | 1989 | 4.691 | | 1988 | 4.763 | | 1987 | 4.864 | | 1986 | 4.989 | | 1985 | 5.128 | | 1984 | 5.266 | | 1983 | 5.385 | | 1982 | 5.475 | | 1981 | 5.532 | | 1980 | 5.553 | | 1979 | 5.541 | | 1978 | 5.509 | | 1977 | 5.471 | | 1976 | 5.438 | | 1975 | 5.426 | | 1974 | 5.449 | | 1973 | 5.512 | | 1972 | 5.616 | | 1971 | 5.761 | | 1970 | 5.941 | | 1969 | 6.15 | | 1968 | 6.371 | | 1967 | 6.59 | | 1966 | 6.596 | | 1965 | 6.662 | | 1964 | 6.734 | | 1963 | 6.774 | | 1962 | 6.804 | | 1961 | 6.844 | | 1960 | 6.885 |
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