| Year | Value | | 2021 | 2.711 | | 2020 | 2.754 | | 2019 | 2.801 | | 2018 | 2.84 | | 2017 | 2.893 | | 2016 | 2.942 | | 2015 | 2.971 | | 2014 | 3.032 | | 2013 | 3.087 | | 2012 | 3.13 | | 2011 | 3.18 | | 2010 | 3.236 | | 2009 | 3.295 | | 2008 | 3.345 | | 2007 | 3.409 | | 2006 | 3.489 | | 2005 | 3.598 | | 2004 | 3.706 | | 2003 | 3.82 | | 2002 | 3.961 | | 2001 | 4.116 | | 2000 | 4.279 | | 1999 | 4.395 | | 1998 | 4.479 | | 1997 | 4.55 | | 1996 | 4.617 | | 1995 | 4.655 | | 1994 | 4.716 | | 1993 | 4.766 | | 1992 | 4.82 | | 1991 | 4.882 | | 1990 | 4.958 | | 1989 | 5.052 | | 1988 | 5.167 | | 1987 | 5.299 | | 1986 | 5.446 | | 1985 | 5.6 | | 1984 | 5.752 | | 1983 | 5.894 | | 1982 | 6.02 | | 1981 | 6.129 | | 1980 | 6.223 | | 1979 | 6.311 | | 1978 | 6.398 | | 1977 | 6.49 | | 1976 | 6.586 | | 1975 | 6.682 | | 1974 | 6.77 | | 1973 | 6.842 | | 1972 | 6.894 | | 1971 | 6.925 | | 1970 | 6.938 | | 1969 | 6.889 | | 1968 | 6.86 | | 1967 | 6.815 | | 1966 | 6.78 | | 1965 | 6.733 | | 1964 | 6.734 | | 1963 | 6.712 | | 1962 | 6.7 | | 1961 | 6.697 | | 1960 | 6.686 |
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