| Year | Value | | 2021 | 3.303 | | 2020 | 3.349 | | 2019 | 3.404 | | 2018 | 3.462 | | 2017 | 3.511 | | 2016 | 3.574 | | 2015 | 3.604 | | 2014 | 3.635 | | 2013 | 3.653 | | 2012 | 3.676 | | 2011 | 3.658 | | 2010 | 3.63 | | 2009 | 3.613 | | 2008 | 3.587 | | 2007 | 3.552 | | 2006 | 3.535 | | 2005 | 3.56 | | 2004 | 3.58 | | 2003 | 3.619 | | 2002 | 3.716 | | 2001 | 3.827 | | 2000 | 3.975 | | 1999 | 4.11 | | 1998 | 4.152 | | 1997 | 4.154 | | 1996 | 4.212 | | 1995 | 4.426 | | 1994 | 4.671 | | 1993 | 4.891 | | 1992 | 5.067 | | 1991 | 5.225 | | 1990 | 5.321 | | 1989 | 5.47 | | 1988 | 5.58 | | 1987 | 5.672 | | 1986 | 5.767 | | 1985 | 5.854 | | 1984 | 5.929 | | 1983 | 5.999 | | 1982 | 6.075 | | 1981 | 6.164 | | 1980 | 6.246 | | 1979 | 6.316 | | 1978 | 6.384 | | 1977 | 6.446 | | 1976 | 6.496 | | 1975 | 6.544 | | 1974 | 6.564 | | 1973 | 6.557 | | 1972 | 6.548 | | 1971 | 6.553 | | 1970 | 6.545 | | 1969 | 6.549 | | 1968 | 6.535 | | 1967 | 6.51 | | 1966 | 6.488 | | 1965 | 6.461 | | 1964 | 6.433 | | 1963 | 6.402 | | 1962 | 6.342 | | 1961 | 6.272 | | 1960 | 6.205 |
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