| Year | Value | | 2021 | 2.791 | | 2020 | 2.836 | | 2019 | 2.883 | | 2018 | 2.903 | | 2017 | 2.91 | | 2016 | 2.93 | | 2015 | 2.942 | | 2014 | 2.951 | | 2013 | 2.964 | | 2012 | 2.981 | | 2011 | 2.999 | | 2010 | 3.023 | | 2009 | 3.045 | | 2008 | 3.066 | | 2007 | 3.082 | | 2006 | 3.091 | | 2005 | 3.08 | | 2004 | 3.073 | | 2003 | 3.11 | | 2002 | 3.181 | | 2001 | 3.261 | | 2000 | 3.312 | | 1999 | 3.342 | | 1998 | 3.396 | | 1997 | 3.458 | | 1996 | 3.6 | | 1995 | 3.803 | | 1994 | 4.009 | | 1993 | 4.13 | | 1992 | 4.229 | | 1991 | 4.339 | | 1990 | 4.487 | | 1989 | 4.651 | | 1988 | 4.852 | | 1987 | 5.06 | | 1986 | 5.27 | | 1985 | 5.427 | | 1984 | 5.582 | | 1983 | 5.746 | | 1982 | 5.933 | | 1981 | 6.128 | | 1980 | 6.253 | | 1979 | 6.358 | | 1978 | 6.435 | | 1977 | 6.495 | | 1976 | 6.556 | | 1975 | 6.578 | | 1974 | 6.593 | | 1973 | 6.593 | | 1972 | 6.593 | | 1971 | 6.592 | | 1970 | 6.617 | | 1969 | 6.639 | | 1968 | 6.668 | | 1967 | 6.678 | | 1966 | 6.692 | | 1965 | 6.687 | | 1964 | 6.688 | | 1963 | 6.632 | | 1962 | 6.571 | | 1961 | 6.597 | | 1960 | 6.628 |
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