| Year | Value | | 2021 | 15.395808233045 | | 2020 | 13.702045498594 | | 2019 | 11.981800061265 | | 2018 | 12.117400321368 | | 2017 | 11.976402434695 | | 2016 | 11.617163059291 | | 2015 | 11.838089597341 | | 2014 | 11.375115818527 | | 2013 | 11.410561522975 | | 2012 | 11.496728860732 | | 2011 | 11.316723165709 | | 2010 | 11.429483646209 | | 2009 | 11.434333580072 | | 2008 | 11.37212513608 | | 2007 | 11.389411086626 | | 2006 | 11.28587384576 | | 2005 | 11.344985860543 | | 2004 | 11.116778891258 | | 2003 | 11.3194437927 | | 2002 | 11.164449861145 | | 2001 | 11.003862362453 | | 2000 | 11.006923362519 | | 1999 | 11.25576409562 | | 1998 | 11.277389933791 | | 1997 | 11.409014228243 | | 1996 | 11.52005716444 | | 1995 | 11.494848563381 | | 1994 | 11.433737367957 | | 1993 | 11.48442673267 | | 1992 | 11.421632973805 | | 1991 | 11.414637571303 | | 1990 | 11.175143347044 | | 1989 | 11.048832695076 | | 1988 | 10.968745417021 | | 1987 | 11.104473640918 | | 1986 | 11.042729458105 | | 1985 | 11.297376284986 | | 1984 | 11.029239070596 | | 1983 | 10.949299388131 | | 1982 | 10.602249217909 | | 1981 | 10.551259708447 | | 1980 | 10.939840379858 | | 1979 | 10.374817309553 | | 1978 | 10.4360201445 | | 1977 | 10.172375767758 | | 1976 | 10.073558682142 | | 1975 | 9.9730658776918 | | 1974 | 9.6654065743982 | | 1973 | 9.8220876748594 | | 1972 | 9.5275392674769 | | 1971 | 9.8559595218793 | | 1970 | 9.6254116919966 | | 1969 | 9.7114851526718 | | 1968 | 9.1975386565133 | | 1967 | 9.0213721373981 | | 1966 | 8.5130687403418 | | 1965 | 8.679034379862 | | 1964 | 8.5057399011432 | | 1963 | 8.5112284209329 | | 1962 | 9.1148285500907 | | 1961 | 8.4950225103368 | | 1960 | 8.632179424896 |
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