| Year | Value | | 2021 | 6.3289301785455 | | 2020 | 6.4450573377849 | | 2019 | 6.43054144288 | | 2018 | 6.5466686021193 | | 2017 | 6.6918275511685 | | 2016 | 6.6918275511685 | | 2015 | 6.5757003919292 | | 2014 | 6.7525039918711 | | 2013 | 6.8811148207287 | | 2012 | 7.0055160400639 | | 2011 | 6.6648279866454 | | 2010 | 6.3701553200755 | | 2009 | 6.7149078240673 | | 2008 | 7.0496443605748 | | 2007 | 6.7564232834954 | | 2006 | 6.7800841921904 | | 2005 | 7.0937726810858 | | 2004 | 7.572216577152 | | 2003 | 7.7112788503411 | | 2002 | 7.52213673973 | | 2001 | 7.3914936855857 | | 2000 | 7.0852083030919 | | 1999 | 6.778922920598 | | 1998 | 6.4726375381042 | | 1997 | 6.1663521556104 | | 1996 | 5.8600667731166 | | 1995 | 5.7570039192916 | | 1994 | 5.8107127304398 | | 1993 | 5.8629699520975 | | 1992 | 5.7032951081434 | | 1991 | 5.6858760342575 | | 1990 | 5.7279721294818 | | 1989 | 6.0153868485992 | | 1988 | 6.1126433444622 | | 1987 | 6.3579619683554 | | 1986 | 6.7629554362026 | | 1985 | 7.3247205690231 | | 1984 | 7.4669763390913 | | 1983 | 7.2216577151981 | | 1982 | 7.5076208448251 | | 1981 | 7.7093917840035 | | 1980 | 8.0345478298737 | | 1979 | 7.52213673973 | | 1978 | 7.3639134852664 | | 1977 | 7.2985919581942 | | 1976 | 6.8805341849325 | | 1975 | 6.6410219190013 | | 1974 | 6.8297285527653 | | 1973 | 6.9995645231529 | | 1972 | 7.3987516330382 | | 1971 | 7.7732617215851 | | 1970 | 7.7253592683989 | | 1969 | 7.4800406445057 | | 1968 | 7.6861663521556 | | 1967 | 7.6658440992887 | | 1966 | 7.4481056757149 | | 1965 | 8.2348671795616 | | 1964 | 8.5005080563217 | | 1963 | 8.9287269560168 | | 1962 | 9.3888808245028 | | 1961 | 9.4585571200464 |
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