| Year | Value | | 2021 | 5.608 | | 2020 | 4.945 | | 2019 | 4.315 | | 2018 | 4.257 | | 2017 | 4.221 | | 2016 | 4.183 | | 2015 | 4.147 | | 2014 | 4.16 | | 2013 | 4.198 | | 2012 | 4.221 | | 2011 | 4.271 | | 2010 | 4.348 | | 2009 | 4.371 | | 2008 | 4.5 | | 2007 | 4.5 | | 2006 | 4.525 | | 2005 | 4.573 | | 2004 | 4.674 | | 2003 | 4.781 | | 2002 | 4.848 | | 2001 | 4.992 | | 2000 | 5.141 | | 1999 | 5.26 | | 1998 | 7.784 | | 1997 | 5.56 | | 1996 | 5.728 | | 1995 | 5.898 | | 1994 | 6.176 | | 1993 | 6.388 | | 1992 | 6.546 | | 1991 | 6.736 | | 1990 | 7.044 | | 1989 | 7.329 | | 1988 | 7.685 | | 1987 | 7.841 | | 1986 | 8.056 | | 1985 | 8.339 | | 1984 | 8.541 | | 1983 | 8.813 | | 1982 | 9.129 | | 1981 | 9.401 | | 1980 | 9.686 | | 1979 | 9.988 | | 1978 | 10.278 | | 1977 | 10.57 | | 1976 | 10.853 | | 1975 | 11.109 | | 1974 | 14.265 | | 1973 | 12.967 | | 1972 | 12.425 | | 1971 | 12.856 | | 1970 | 13.245 | | 1969 | 13.661 | | 1968 | 14.087 | | 1967 | 14.615 | | 1966 | 15.086 | | 1965 | 15.61 | | 1964 | 16.144 | | 1963 | 16.643 | | 1962 | 17.185 | | 1961 | 17.976 | | 1960 | 18.449 |
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