| Year | Value | | 2021 | 4.308 | | 2020 | 4.379 | | 2019 | 4.451 | | 2018 | 4.536 | | 2017 | 4.614 | | 2016 | 4.707 | | 2015 | 4.793 | | 2014 | 4.899 | | 2013 | 5.026 | | 2012 | 5.146 | | 2011 | 5.271 | | 2010 | 5.363 | | 2009 | 5.452 | | 2008 | 5.537 | | 2007 | 5.609 | | 2006 | 5.686 | | 2005 | 5.71 | | 2004 | 5.715 | | 2003 | 5.726 | | 2002 | 5.792 | | 2001 | 5.862 | | 2000 | 5.926 | | 1999 | 5.987 | | 1998 | 6.049 | | 1997 | 6.117 | | 1996 | 6.183 | | 1995 | 6.241 | | 1994 | 6.274 | | 1993 | 6.318 | | 1992 | 6.364 | | 1991 | 6.447 | | 1990 | 6.533 | | 1989 | 6.605 | | 1988 | 6.675 | | 1987 | 6.773 | | 1986 | 6.851 | | 1985 | 6.921 | | 1984 | 6.977 | | 1983 | 7.022 | | 1982 | 7.063 | | 1981 | 7.103 | | 1980 | 7.154 | | 1979 | 7.232 | | 1978 | 7.285 | | 1977 | 7.333 | | 1976 | 7.366 | | 1975 | 7.391 | | 1974 | 7.376 | | 1973 | 7.393 | | 1972 | 7.392 | | 1971 | 7.383 | | 1970 | 7.367 | | 1969 | 7.349 | | 1968 | 7.332 | | 1967 | 7.317 | | 1966 | 7.304 | | 1965 | 7.291 | | 1964 | 7.274 | | 1963 | 7.249 | | 1962 | 7.214 | | 1961 | 7.169 | | 1960 | 7.115 |
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