| Year | Value | | 2021 | 7.884 | | 2020 | 6.752 | | 2019 | 6.145 | | 2018 | 6.04 | | 2017 | 5.974 | | 2016 | 5.883 | | 2015 | 5.811 | | 2014 | 5.715 | | 2013 | 5.591 | | 2012 | 5.519 | | 2011 | 5.441 | | 2010 | 5.329 | | 2009 | 5.271 | | 2008 | 5.169 | | 2007 | 5.076 | | 2006 | 5.007 | | 2005 | 4.937 | | 2004 | 4.939 | | 2003 | 4.884 | | 2002 | 4.863 | | 2001 | 4.847 | | 2000 | 4.773 | | 1999 | 4.725 | | 1998 | 4.751 | | 1997 | 4.774 | | 1996 | 4.887 | | 1995 | 4.979 | | 1994 | 5.074 | | 1993 | 5.171 | | 1992 | 5.341 | | 1991 | 5.366 | | 1990 | 5.5 | | 1989 | 5.659 | | 1988 | 5.713 | | 1987 | 5.97 | | 1986 | 6.242 | | 1985 | 6.481 | | 1984 | 6.807 | | 1983 | 7.062 | | 1982 | 7.33 | | 1981 | 7.607 | | 1980 | 7.88 | | 1979 | 8.233 | | 1978 | 8.663 | | 1977 | 9.073 | | 1976 | 9.612 | | 1975 | 10.104 | | 1974 | 10.719 | | 1973 | 11.319 | | 1972 | 11.968 | | 1971 | 12.796 | | 1970 | 13.535 | | 1969 | 14.514 | | 1968 | 15.321 | | 1967 | 16.235 | | 1966 | 17.168 | | 1965 | 17.886 | | 1964 | 18.681 | | 1963 | 19.371 | | 1962 | 20.205 | | 1961 | 21.238 | | 1960 | 21.537 |
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