| Year | Value | | 2021 | 8.5443127964246 | | 2020 | 7.6809871838647 | | 2019 | 7.1530425218805 | | 2018 | 7.1504098406719 | | 2017 | 7.1521539019202 | | 2016 | 7.193072419005 | | 2015 | 7.2367176348673 | | 2014 | 7.2916956959243 | | 2013 | 7.3613992940539 | | 2012 | 7.439701137722 | | 2011 | 7.5210523049014 | | 2010 | 7.6178627725822 | | 2009 | 7.6609352814019 | | 2008 | 7.7601322972844 | | 2007 | 7.7507858435822 | | 2006 | 7.7722721333653 | | 2005 | 7.8038276510916 | | 2004 | 7.8693391375966 | | 2003 | 7.9136508824745 | | 2002 | 7.9670750091009 | | 2001 | 8.0259184591958 | | 2000 | 8.0892213837102 | | 1999 | 8.1263569776512 | | 1998 | 8.2017943134536 | | 1997 | 8.313490874883 | | 1996 | 8.4441424189937 | | 1995 | 8.5576490571124 | | 1994 | 8.6306178398243 | | 1993 | 8.7156608844197 | | 1992 | 8.7020480177173 | | 1991 | 8.8069221994574 | | 1990 | 8.867281389244 | | 1989 | 8.9176068539044 | | 1988 | 9.0932873582062 | | 1987 | 9.2224675725707 | | 1986 | 9.3889995999127 | | 1985 | 9.5746954530945 | | 1984 | 9.7481095710748 | | 1983 | 9.9070219681201 | | 1982 | 9.9724635431928 | | 1981 | 10.054795655227 | | 1980 | 10.198069444406 | | 1979 | 10.285276201578 | | 1978 | 10.462188772085 | | 1977 | 10.808453479957 | | 1976 | 11.229755591867 | | 1975 | 11.408827506133 | | 1974 | 11.497804715109 | | 1973 | 11.574962194849 | | 1972 | 11.969317679298 | | 1971 | 12.668280504817 | | 1970 | 12.43597186963 | | 1969 | 12.601008155865 | | 1968 | 12.812357247018 | | 1967 | 13.242579774528 | | 1966 | 13.55410064129 | | 1965 | 14.143188190885 | | 1964 | 14.436326509933 | | 1963 | 14.21472107737 | | 1962 | 14.42033251388 | | 1961 | 15.88021386803 | | 1960 | 19.620107208782 |
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