| Year | Value | | 2021 | 2.397 | | 2020 | 2.418 | | 2019 | 2.44 | | 2018 | 2.462 | | 2017 | 2.485 | | 2016 | 2.509 | | 2015 | 2.534 | | 2014 | 2.558 | | 2013 | 2.582 | | 2012 | 2.606 | | 2011 | 2.63 | | 2010 | 2.655 | | 2009 | 2.684 | | 2008 | 2.718 | | 2007 | 2.755 | | 2006 | 2.797 | | 2005 | 2.84 | | 2004 | 2.883 | | 2003 | 2.924 | | 2002 | 2.961 | | 2001 | 2.992 | | 2000 | 3.017 | | 1999 | 3.036 | | 1998 | 3.05 | | 1997 | 3.061 | | 1996 | 3.069 | | 1995 | 3.074 | | 1994 | 3.074 | | 1993 | 3.071 | | 1992 | 3.066 | | 1991 | 3.063 | | 1990 | 3.069 | | 1989 | 3.089 | | 1988 | 3.127 | | 1987 | 3.186 | | 1986 | 3.263 | | 1985 | 3.354 | | 1984 | 3.452 | | 1983 | 3.548 | | 1982 | 3.64 | | 1981 | 3.728 | | 1980 | 3.817 | | 1979 | 3.915 | | 1978 | 4.031 | | 1977 | 4.169 | | 1976 | 4.327 | | 1975 | 4.503 | | 1974 | 4.689 | | 1973 | 4.877 | | 1972 | 5.057 | | 1971 | 5.226 | | 1970 | 5.38 | | 1969 | 5.517 | | 1968 | 5.642 | | 1967 | 5.756 | | 1966 | 5.86 | | 1965 | 5.957 | | 1964 | 6.05 | | 1963 | 6.14 | | 1962 | 6.226 | | 1961 | 6.305 | | 1960 | 6.372 |
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