| Year | Value | | 2021 | 14.455445133099 | | 2020 | 14.35009068745 | | 2019 | 14.347349025736 | | 2018 | 14.343038477173 | | 2017 | 14.150157367025 | | 2016 | 14.040920230216 | | 2015 | 13.930395921249 | | 2014 | 13.819646394592 | | 2013 | 13.708473488615 | | 2012 | 13.597593884054 | | 2011 | 13.462285784109 | | 2010 | 12.905701652332 | | 2009 | 12.358798292833 | | 2008 | 12.05598176258 | | 2007 | 12.438076985439 | | 2006 | 11.86943387756 | | 2005 | 11.361866612331 | | 2004 | 10.955191929722 | | 2003 | 10.928 | | 2002 | 9.853 | | 2001 | 9.862 | | 2000 | 9.872 | | 1999 | 9.881 | | 1998 | 9.891 | | 1997 | 9.9 | | 1996 | 9.927 | | 1995 | 9.94 | | 1994 | 9.932 | | 1993 | 10 | | 1992 | 9.6730245231608 | | 1991 | 9.6730245231608 | | 1990 | 9.763851044505 | | 1989 | 9.9000908265213 | | 1988 | 10.08174386921 | | 1987 | 10.263396911898 | | 1986 | 10.535876475931 | | 1985 | 10.762942779292 | | 1984 | 10.899182561308 | | 1983 | 11.262488646685 | | 1982 | 11.444141689373 | | 1981 | 11.625794732062 | | 1980 | 11.80744777475 | | 1979 | 11.902815622162 | | 1978 | 11.861943687557 | | 1977 | 11.834695731154 | | 1976 | 11.80744777475 | | 1975 | 11.80744777475 | | 1974 | 11.80744777475 | | 1973 | 11.80744777475 | | 1972 | 11.80744777475 | | 1971 | 11.598546775658 | | 1970 | 11.380563124432 | | 1969 | 11.380563124432 | | 1968 | 11.126248864669 | | 1967 | 11.035422343324 | | 1966 | 10.962761126249 | | 1965 | 10.894641235241 | | 1964 | 10.564032697548 | | 1963 | 10.564032697548 | | 1962 | 10.407811080836 | | 1961 | 9.9909173478656 |
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