| Year | Value | | 2021 | 4.159 | | 2020 | 4.243 | | 2019 | 4.323 | | 2018 | 4.343 | | 2017 | 4.39 | | 2016 | 4.469 | | 2015 | 4.531 | | 2014 | 4.609 | | 2013 | 4.724 | | 2012 | 4.883 | | 2011 | 5.05 | | 2010 | 5.217 | | 2009 | 5.39 | | 2008 | 5.571 | | 2007 | 5.74 | | 2006 | 5.853 | | 2005 | 5.973 | | 2004 | 6.107 | | 2003 | 6.24 | | 2002 | 6.374 | | 2001 | 6.463 | | 2000 | 6.56 | | 1999 | 6.646 | | 1998 | 6.753 | | 1997 | 6.85 | | 1996 | 6.923 | | 1995 | 7.008 | | 1994 | 7.076 | | 1993 | 7.125 | | 1992 | 7.157 | | 1991 | 7.207 | | 1990 | 7.238 | | 1989 | 7.271 | | 1988 | 7.303 | | 1987 | 7.338 | | 1986 | 7.359 | | 1985 | 7.378 | | 1984 | 7.4 | | 1983 | 7.383 | | 1982 | 7.372 | | 1981 | 7.357 | | 1980 | 7.334 | | 1979 | 7.31 | | 1978 | 7.296 | | 1977 | 7.255 | | 1976 | 7.188 | | 1975 | 7.143 | | 1974 | 7.121 | | 1973 | 7.094 | | 1972 | 7.06 | | 1971 | 7.02 | | 1970 | 6.978 | | 1969 | 6.937 | | 1968 | 6.903 | | 1967 | 6.88 | | 1966 | 6.867 | | 1965 | 6.864 | | 1964 | 6.867 | | 1963 | 6.872 | | 1962 | 6.875 | | 1961 | 6.877 | | 1960 | 6.88 |
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