| Year | Value | | 2021 | 11.628587786299 | | 2020 | 11.633259401711 | | 2019 | 11.636713230327 | | 2018 | 11.742944266657 | | 2017 | 11.835558010101 | | 2016 | 11.860185063635 | | 2015 | 11.861996705955 | | 2014 | 11.848606783886 | | 2013 | 11.90752607637 | | 2012 | 11.969300824595 | | 2011 | 12.04624962772 | | 2010 | 12.121004417234 | | 2009 | 12.194278612227 | | 2008 | 12.184214796557 | | 2007 | 12.146221518189 | | 2006 | 12.128592896847 | | 2005 | 12.081022433307 | | 2004 | 12.062271366053 | | 2003 | 11.993160747042 | | 2002 | 11.939598344018 | | 2001 | 11.874928565904 | | 2000 | 11.757735621219 | | 1999 | 11.717975440134 | | 1998 | 11.642228848445 | | 1997 | 11.609418827446 | | 1996 | 11.580715946601 | | 1995 | 11.548787297289 | | 1994 | 11.615141046908 | | 1993 | 11.719973469651 | | 1992 | 11.828234784914 | | 1991 | 11.965503526082 | | 1990 | 12.030119960652 | | 1989 | 11.97030398863 | | 1988 | 11.903956243099 | | 1987 | 11.846131205887 | | 1986 | 11.770149094429 | | 1985 | 11.681872803506 | | 1984 | 11.177567760009 | | 1983 | 10.751050486512 | | 1982 | 10.389651854627 | | 1981 | 10.056594211166 | | 1980 | 9.9959372733044 | | 1979 | 9.9824409962907 | | 1978 | 9.9739553432249 | | 1977 | 9.950742845858 | | 1976 | 9.898083348676 | | 1975 | 9.8590928499532 | | 1974 | 9.9239792066522 | | 1973 | 9.8347971592029 | | 1972 | 9.8361691351542 | | 1971 | 9.8100264875095 | | 1970 | 9.8828007099848 | | 1969 | 9.8899246992277 | | 1968 | 9.9154462038449 | | 1967 | 9.9349396692526 | | 1966 | 9.9567613219232 | | 1965 | 9.9707800615768 | | 1964 | 9.9765380386003 | | 1963 | 9.9657889355508 | | 1962 | 9.9648518825709 | | 1961 | 9.9631429541807 |
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