| Year | Value | | 2021 | 11.572244083941 | | 2020 | 11.576897754655 | | 2019 | 11.580377017947 | | 2018 | 11.687414257003 | | 2017 | 11.78073095317 | | 2016 | 11.804599707396 | | 2015 | 11.805795870563 | | 2014 | 11.79135887684 | | 2013 | 11.849778585337 | | 2012 | 11.912024029098 | | 2011 | 11.989556678287 | | 2010 | 12.061727538848 | | 2009 | 12.135556629998 | | 2008 | 12.125416585819 | | 2007 | 12.090286237339 | | 2006 | 12.072524492738 | | 2005 | 12.024582048588 | | 2004 | 12.008831486284 | | 2003 | 11.939197423824 | | 2002 | 11.8852051882 | | 2001 | 11.820046121946 | | 2000 | 11.701963054423 | | 1999 | 11.661718137885 | | 1998 | 11.585394402511 | | 1997 | 11.552334388792 | | 1996 | 11.523412809177 | | 1995 | 11.484913042197 | | 1994 | 11.551772407447 | | 1993 | 11.66373146237 | | 1992 | 11.772817663003 | | 1991 | 11.911132308484 | | 1990 | 11.976995921154 | | 1989 | 11.916914001445 | | 1988 | 11.850060683581 | | 1987 | 11.791795017395 | | 1986 | 11.715233919371 | | 1985 | 11.626284959768 | | 1984 | 11.118137092743 | | 1983 | 10.68836974147 | | 1982 | 10.324217238324 | | 1981 | 9.9886216830078 | | 1980 | 9.9275025369831 | | 1979 | 9.919282565895 | | 1978 | 9.9107322518315 | | 1977 | 9.8873428743556 | | 1976 | 9.834282109811 | | 1975 | 9.7949945020114 | | 1974 | 9.8603753757078 | | 1973 | 9.7705139212639 | | 1972 | 9.7750606604307 | | 1971 | 9.7487188059555 | | 1970 | 9.8220475676035 | | 1969 | 9.829225841605 | | 1968 | 9.8549418199462 | | 1967 | 9.8745838254525 | | 1966 | 9.8965717589958 | | 1965 | 9.9106973213679 | | 1964 | 9.9164991741445 | | 1963 | 9.9056681629892 | | 1962 | 9.9047239696708 | | 1961 | 9.9030020192557 |
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