| Year | Value | | 2021 | 4.005 | | 2020 | 4.094 | | 2019 | 4.172 | | 2018 | 4.263 | | 2017 | 4.358 | | 2016 | 4.531 | | 2015 | 4.671 | | 2014 | 4.802 | | 2013 | 4.937 | | 2012 | 5.003 | | 2011 | 5.047 | | 2010 | 5.061 | | 2009 | 5.152 | | 2008 | 5.274 | | 2007 | 5.399 | | 2006 | 5.417 | | 2005 | 5.454 | | 2004 | 5.5 | | 2003 | 5.566 | | 2002 | 5.612 | | 2001 | 5.63 | | 2000 | 5.72 | | 1999 | 5.793 | | 1998 | 5.898 | | 1997 | 6.008 | | 1996 | 6.092 | | 1995 | 6.172 | | 1994 | 6.258 | | 1993 | 6.323 | | 1992 | 6.402 | | 1991 | 6.452 | | 1990 | 6.507 | | 1989 | 6.663 | | 1988 | 6.693 | | 1987 | 6.717 | | 1986 | 6.734 | | 1985 | 6.739 | | 1984 | 6.727 | | 1983 | 6.694 | | 1982 | 6.64 | | 1981 | 6.569 | | 1980 | 6.487 | | 1979 | 6.401 | | 1978 | 6.32 | | 1977 | 6.249 | | 1976 | 6.192 | | 1975 | 6.15 | | 1974 | 6.12 | | 1973 | 6.098 | | 1972 | 6.078 | | 1971 | 6.06 | | 1970 | 6.041 | | 1969 | 6.024 | | 1968 | 6.008 | | 1967 | 5.994 | | 1966 | 5.982 | | 1965 | 5.971 | | 1964 | 5.961 | | 1963 | 5.951 | | 1962 | 5.941 | | 1961 | 5.931 | | 1960 | 5.921 |
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