| Year | Value | | 2021 | 2.363 | | 2020 | 2.394 | | 2019 | 2.427 | | 2018 | 2.46 | | 2017 | 2.496 | | 2016 | 2.536 | | 2015 | 2.581 | | 2014 | 2.635 | | 2013 | 2.699 | | 2012 | 2.774 | | 2011 | 2.86 | | 2010 | 2.957 | | 2009 | 3.063 | | 2008 | 3.177 | | 2007 | 3.296 | | 2006 | 3.419 | | 2005 | 3.545 | | 2004 | 3.677 | | 2003 | 3.815 | | 2002 | 3.956 | | 2001 | 4.098 | | 2000 | 4.237 | | 1999 | 4.361 | | 1998 | 4.448 | | 1997 | 4.527 | | 1996 | 4.639 | | 1995 | 4.758 | | 1994 | 4.891 | | 1993 | 4.995 | | 1992 | 5.089 | | 1991 | 5.183 | | 1990 | 5.291 | | 1989 | 5.395 | | 1988 | 5.496 | | 1987 | 5.604 | | 1986 | 5.707 | | 1985 | 5.806 | | 1984 | 5.921 | | 1983 | 6.048 | | 1982 | 6.164 | | 1981 | 6.282 | | 1980 | 6.393 | | 1979 | 6.499 | | 1978 | 6.604 | | 1977 | 6.701 | | 1976 | 6.794 | | 1975 | 6.858 | | 1974 | 6.921 | | 1973 | 7.008 | | 1972 | 7.082 | | 1971 | 7.124 | | 1970 | 7.17 | | 1969 | 7.229 | | 1968 | 7.289 | | 1967 | 7.334 | | 1966 | 7.384 | | 1965 | 7.442 | | 1964 | 7.443 | | 1963 | 7.441 | | 1962 | 7.443 | | 1961 | 7.448 | | 1960 | 7.458 |
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