| Year | Value | | 2021 | 12.516946532545 | | 2020 | 11.338996647021 | | 2019 | 9.6666927394125 | | 2018 | 9.7906443267032 | | 2017 | 9.790261228043 | | 2016 | 10.035593729891 | | 2015 | 10.127441709589 | | 2014 | 10.165170316778 | | 2013 | 10.130850357753 | | 2012 | 10.337142340637 | | 2011 | 10.499103373935 | | 2010 | 10.848684262469 | | 2009 | 10.832627304685 | | 2008 | 11.205382644866 | | 2007 | 11.320143155803 | | 2006 | 11.542997308489 | | 2005 | 12.043466383294 | | 2004 | 11.853470112012 | | 2003 | 12.1338539205 | | 2002 | 12.067901604766 | | 2001 | 11.76934289505 | | 2000 | 11.744603970451 | | 1999 | 11.494277326523 | | 1998 | 11.040899237076 | | 1997 | 11.293597333037 | | 1996 | 11.575917058147 | | 1995 | 11.997480684068 | | 1994 | 12.195777028313 | | 1993 | 11.767308527431 | | 1992 | 10.639993142837 | | 1991 | 10.04875276971 | | 1990 | 9.8375537725009 | | 1989 | 9.5880323418209 | | 1988 | 9.7601738958743 | | 1987 | 9.6322441568446 | | 1986 | 9.5756716939906 | | 1985 | 10.239504538359 | | 1984 | 10.41207116077 | | 1983 | 10.14902014306 | | 1982 | 9.9710893154446 | | 1981 | 10.099827527552 | | 1980 | 10.298470096961 | | 1979 | 10.170177327806 | | 1978 | 10.192732731825 | | 1977 | 10.127915892675 | | 1976 | 10.072298175154 | | 1975 | 9.9856321460899 | | 1974 | 9.8012498938928 | | 1973 | 9.7802313153578 | | 1972 | 9.7688789240862 | | 1971 | 9.6917434446345 | | 1970 | 9.7088143622497 | | 1969 | 9.7353365158564 | | 1968 | 9.4663841875729 | | 1967 | 9.4353496623987 | | 1966 | 9.298978606218 | | 1965 | 9.3194943154923 | | 1964 | 9.2105950180626 | | 1963 | 9.4660870651041 | | 1962 | 9.7242266243441 | | 1961 | 9.6276090975179 | | 1960 | 9.7400412486145 |
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