| Year | Value | | 2021 | 4.399 | | 2020 | 4.489 | | 2019 | 4.576 | | 2018 | 4.669 | | 2017 | 4.789 | | 2016 | 4.926 | | 2015 | 5.022 | | 2014 | 5.049 | | 2013 | 5.116 | | 2012 | 5.195 | | 2011 | 5.296 | | 2010 | 5.397 | | 2009 | 5.508 | | 2008 | 5.558 | | 2007 | 5.617 | | 2006 | 5.659 | | 2005 | 5.698 | | 2004 | 5.719 | | 2003 | 5.737 | | 2002 | 5.793 | | 2001 | 5.85 | | 2000 | 5.936 | | 1999 | 6.009 | | 1998 | 6.076 | | 1997 | 6.162 | | 1996 | 6.249 | | 1995 | 6.335 | | 1994 | 6.409 | | 1993 | 6.492 | | 1992 | 6.553 | | 1991 | 6.592 | | 1990 | 6.627 | | 1989 | 6.655 | | 1988 | 6.664 | | 1987 | 6.667 | | 1986 | 6.641 | | 1985 | 6.61 | | 1984 | 6.597 | | 1983 | 6.586 | | 1982 | 6.575 | | 1981 | 6.556 | | 1980 | 6.541 | | 1979 | 6.537 | | 1978 | 6.467 | | 1977 | 6.434 | | 1976 | 6.4 | | 1975 | 6.367 | | 1974 | 6.336 | | 1973 | 6.308 | | 1972 | 6.283 | | 1971 | 6.262 | | 1970 | 6.243 | | 1969 | 6.228 | | 1968 | 6.214 | | 1967 | 6.202 | | 1966 | 6.19 | | 1965 | 6.178 | | 1964 | 6.166 | | 1963 | 6.153 | | 1962 | 6.14 | | 1961 | 6.126 | | 1960 | 6.112 |
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