| Year | Value | | 2021 | 6.2245825107492 | | 2020 | 6.2328815199923 | | 2019 | 6.2367354402862 | | 2018 | 6.2409123414994 | | 2017 | 6.2731236286879 | | 2016 | 6.2988288250953 | | 2015 | 6.3224226109094 | | 2014 | 6.3355720053803 | | 2013 | 6.3549415880652 | | 2012 | 6.3676582469128 | | 2011 | 6.3929442674014 | | 2010 | 6.428541092361 | | 2009 | 6.450881168715 | | 2008 | 6.4189667739236 | | 2007 | 6.4599644964633 | | 2006 | 6.4945793400447 | | 2005 | 6.5967054033771 | | 2004 | 6.4950703307338 | | 2003 | 6.4850050216073 | | 2002 | 6.5166739210541 | | 2001 | 6.5763292897795 | | 2000 | 6.381160490863 | | 1999 | 6.7363922544255 | | 1998 | 6.8272255319086 | | 1997 | 6.864049833591 | | 1996 | 6.8959642283824 | | 1995 | 6.7854913233353 | | 1994 | 6.8174057181267 | | 1993 | 6.8174057181267 | | 1992 | 6.7879462767808 | | 1991 | 6.8419552525816 | | 1990 | 6.974522738638 | | 1989 | 6.994162366202 | | 1988 | 7.0408064816663 | | 1987 | 7.0849956436851 | | 1986 | 7.1291848057039 | | 1985 | 7.1635541539408 | | 1984 | 7.1905586418412 | | 1983 | 7.2101982694051 | | 1982 | 7.2322928504145 | | 1981 | 7.2568423848694 | | 1980 | 7.3034865003337 | | 1979 | 7.3182162210067 | | 1978 | 7.3550405226891 | | 1977 | 7.3403108020161 | | 1976 | 7.3624053830255 | | 1975 | 7.369770243362 | | 1974 | 7.4016846381534 | | 1973 | 7.3992296847079 | | 1972 | 7.4311440794992 | | 1971 | 7.4802431484091 | | 1970 | 7.4851530553 | | 1969 | 7.490062962191 | | 1968 | 7.4311440794992 | | 1967 | 7.6005358672381 | | 1966 | 7.6324502620295 | | 1965 | 7.7601078411949 | | 1964 | 7.990873465071 | | 1963 | 8.0841616959997 | | 1962 | 8.5530578040883 | | 1961 | 8.7126297780452 |
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