| Year | Value | | 2021 | 9.1879250543452 | | 2020 | 9.1611631223989 | | 2019 | 9.1064067865843 | | 2018 | 9.1316004447413 | | 2017 | 9.1543342994475 | | 2016 | 9.1766119546268 | | 2015 | 9.1880927117705 | | 2014 | 9.1651492584069 | | 2013 | 9.1733907285464 | | 2012 | 9.1821214376878 | | 2011 | 9.1790478651397 | | 2010 | 9.1964251536994 | | 2009 | 9.1428384722915 | | 2008 | 9.166959839508 | | 2007 | 9.2014744531083 | | 2006 | 9.1765781251648 | | 2005 | 9.1187853151346 | | 2004 | 9.0808178673726 | | 2003 | 9.0385288830023 | | 2002 | 8.9984019579255 | | 2001 | 8.9884015134221 | | 2000 | 8.9823735385727 | | 1999 | 9.0067780204419 | | 1998 | 8.9943138681046 | | 1997 | 9.0104727164988 | | 1996 | 8.9756240845958 | | 1995 | 9.0017952827321 | | 1994 | 9.0463431037641 | | 1993 | 9.1161959121611 | | 1992 | 9.1951749573573 | | 1991 | 9.3782805891361 | | 1990 | 9.4077508907774 | | 1989 | 9.3872280505523 | | 1988 | 9.3695562717649 | | 1987 | 9.372481922119 | | 1986 | 9.3370286144231 | | 1985 | 9.3176515907152 | | 1984 | 9.0368445348354 | | 1983 | 8.7837166462951 | | 1982 | 8.5817041033417 | | 1981 | 8.4272472527156 | | 1980 | 8.3584756405526 | | 1979 | 8.2799597881737 | | 1978 | 8.176366331496 | | 1977 | 8.0774348002814 | | 1976 | 7.9667744113374 | | 1975 | 7.871266372298 | | 1974 | 7.8656855515578 | | 1973 | 7.7998690850366 | | 1972 | 7.7463337727627 | | 1971 | 7.6511665248827 | | 1970 | 7.616608814397 | | 1969 | 7.6360654537341 | | 1968 | 7.5925749223167 | | 1967 | 7.5259651998491 | | 1966 | 7.497698257052 | | 1965 | 7.4568267167653 | | 1964 | 7.4301230969073 | | 1963 | 7.3903942842861 | | 1962 | 7.3281000595867 | | 1961 | 7.2928089739291 |
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