| Year | Value | | 2021 | 2.01 | | 2020 | 1.999 | | 2019 | 2.106 | | 2018 | 2.345 | | 2017 | 2.36 | | 2016 | 2.394 | | 2015 | 2.438 | | 2014 | 2.491 | | 2013 | 2.53 | | 2012 | 2.555 | | 2011 | 2.585 | | 2010 | 2.606 | | 2009 | 2.701 | | 2008 | 2.799 | | 2007 | 2.874 | | 2006 | 2.982 | | 2005 | 3.129 | | 2004 | 3.207 | | 2003 | 3.321 | | 2002 | 3.465 | | 2001 | 3.554 | | 2000 | 3.634 | | 1999 | 3.714 | | 1998 | 3.785 | | 1997 | 3.854 | | 1996 | 3.852 | | 1995 | 3.948 | | 1994 | 3.999 | | 1993 | 4.198 | | 1992 | 4.348 | | 1991 | 4.483 | | 1990 | 4.698 | | 1989 | 4.762 | | 1988 | 4.783 | | 1987 | 4.82 | | 1986 | 4.922 | | 1985 | 5.02 | | 1984 | 5.187 | | 1983 | 5.393 | | 1982 | 5.597 | | 1981 | 5.684 | | 1980 | 5.849 | | 1979 | 5.992 | | 1978 | 6.109 | | 1977 | 6.195 | | 1976 | 6.25 | | 1975 | 6.278 | | 1974 | 6.287 | | 1973 | 6.285 | | 1972 | 6.284 | | 1971 | 6.288 | | 1970 | 6.299 | | 1969 | 6.316 | | 1968 | 6.337 | | 1967 | 6.358 | | 1966 | 6.379 | | 1965 | 6.4 | | 1964 | 6.42 | | 1963 | 6.44 | | 1962 | 6.46 | | 1961 | 6.48 | | 1960 | 6.5 |
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