| Year | Value | | 2021 | 2.273 | | 2020 | 2.303 | | 2019 | 2.344 | | 2018 | 2.381 | | 2017 | 2.39 | | 2016 | 2.406 | | 2015 | 2.43 | | 2014 | 2.445 | | 2013 | 2.466 | | 2012 | 2.485 | | 2011 | 2.513 | | 2010 | 2.538 | | 2009 | 2.556 | | 2008 | 2.533 | | 2007 | 2.51 | | 2006 | 2.533 | | 2005 | 2.605 | | 2004 | 2.664 | | 2003 | 2.721 | | 2002 | 2.774 | | 2001 | 2.812 | | 2000 | 2.859 | | 1999 | 2.865 | | 1998 | 2.873 | | 1997 | 2.953 | | 1996 | 3.04 | | 1995 | 3.101 | | 1994 | 3.163 | | 1993 | 3.226 | | 1992 | 3.287 | | 1991 | 3.349 | | 1990 | 3.411 | | 1989 | 3.475 | | 1988 | 3.5 | | 1987 | 3.528 | | 1986 | 3.603 | | 1985 | 3.686 | | 1984 | 3.822 | | 1983 | 3.947 | | 1982 | 4.014 | | 1981 | 4.149 | | 1980 | 4.302 | | 1979 | 4.473 | | 1978 | 4.644 | | 1977 | 4.842 | | 1976 | 5.046 | | 1975 | 5.235 | | 1974 | 5.44 | | 1973 | 5.667 | | 1972 | 5.841 | | 1971 | 6.009 | | 1970 | 6.184 | | 1969 | 6.366 | | 1968 | 6.548 | | 1967 | 6.755 | | 1966 | 6.949 | | 1965 | 7.053 | | 1964 | 7.186 | | 1963 | 7.303 | | 1962 | 7.404 | | 1961 | 7.487 | | 1960 | 7.555 |
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