| Year | Value | | 2021 | 6.018 | | 2020 | 5.598 | | 2019 | 5.454 | | 2018 | 5.245 | | 2017 | 5.284 | | 2016 | 5.48 | | 2015 | 5.427 | | 2014 | 5.43 | | 2013 | 5.056 | | 2012 | 5.028 | | 2011 | 5.479 | | 2010 | 4.512 | | 2009 | 4.449 | | 2008 | 4.42 | | 2007 | 4.504 | | 2006 | 4.491 | | 2005 | 4.53 | | 2004 | 4.549 | | 2003 | 4.653 | | 2002 | 4.669 | | 2001 | 4.683 | | 2000 | 4.686 | | 1999 | 4.469 | | 1998 | 4.385 | | 1997 | 4.3 | | 1996 | 4.187 | | 1995 | 4.198 | | 1994 | 4.281 | | 1993 | 4.348 | | 1992 | 4.395 | | 1991 | 4.524 | | 1990 | 4.663 | | 1989 | 4.823 | | 1988 | 4.972 | | 1987 | 5.142 | | 1986 | 5.319 | | 1985 | 5.521 | | 1984 | 5.707 | | 1983 | 5.953 | | 1982 | 6.263 | | 1981 | 6.544 | | 1980 | 6.878 | | 1979 | 7.281 | | 1978 | 7.69 | | 1977 | 8.163 | | 1976 | 8.649 | | 1975 | 9.378 | | 1974 | 10.14 | | 1973 | 10.989 | | 1972 | 11.685 | | 1971 | 12.584 | | 1970 | 13.52 | | 1969 | 14.535 | | 1968 | 15.629 | | 1967 | 16.78 | | 1966 | 17.846 | | 1965 | 18.986 | | 1964 | 20.202 | | 1963 | 21.676 | | 1962 | 22.903 | | 1961 | 24.413 | | 1960 | 25.985 |
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