| Year | Value | | 2021 | 9.987 | | 2020 | 8.026 | | 2019 | 7.444 | | 2018 | 7.499 | | 2017 | 7.472 | | 2016 | 7.475 | | 2015 | 7.431 | | 2014 | 7.459 | | 2013 | 7.538 | | 2012 | 7.513 | | 2011 | 7.455 | | 2010 | 7.399 | | 2009 | 7.321 | | 2008 | 7.288 | | 2007 | 7.16 | | 2006 | 7.297 | | 2005 | 7.414 | | 2004 | 7.402 | | 2003 | 7.453 | | 2002 | 7.351 | | 2001 | 7.286 | | 2000 | 7.399 | | 1999 | 7.525 | | 1998 | 7.638 | | 1997 | 7.896 | | 1996 | 7.977 | | 1995 | 8.023 | | 1994 | 7.916 | | 1993 | 8.031 | | 1992 | 8.218 | | 1991 | 8.266 | | 1990 | 8.358 | | 1989 | 8.42 | | 1988 | 8.458 | | 1987 | 8.479 | | 1986 | 8.509 | | 1985 | 8.533 | | 1984 | 8.526 | | 1983 | 8.528 | | 1982 | 8.52 | | 1981 | 8.506 | | 1980 | 8.502 | | 1979 | 8.541 | | 1978 | 8.577 | | 1977 | 8.63 | | 1976 | 8.675 | | 1975 | 8.73 | | 1974 | 8.784 | | 1973 | 8.848 | | 1972 | 8.894 | | 1971 | 8.969 | | 1970 | 9.03 | | 1969 | 9.091 | | 1968 | 9.176 | | 1967 | 9.284 | | 1966 | 9.416 | | 1965 | 9.571 | | 1964 | 9.77 | | 1963 | 10.013 | | 1962 | 10.298 | | 1961 | 10.637 | | 1960 | 10.915 |
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