| Year | Value | | 2021 | 7.8185529950204 | | 2020 | 7.3838226120831 | | 2019 | 7.2535003915034 | | 2018 | 7.2224307559024 | | 2017 | 7.2039929342029 | | 2016 | 7.1641396608713 | | 2015 | 7.1539392333593 | | 2014 | 7.1642438206662 | | 2013 | 7.16898227858 | | 2012 | 7.1707796833412 | | 2011 | 7.1615974704795 | | 2010 | 7.1170179404533 | | 2009 | 7.0681893117082 | | 2008 | 7.1299243617093 | | 2007 | 6.9537471676739 | | 2006 | 6.8625565784228 | | 2005 | 6.6907048791 | | 2004 | 6.6859581369383 | | 2003 | 6.5738591296453 | | 2002 | 6.6113401317007 | | 2001 | 6.6268514252759 | | 2000 | 6.6532342104345 | | 1999 | 6.6694942746788 | | 1998 | 6.6701181398769 | | 1997 | 6.6787937339039 | | 1996 | 6.7283372676127 | | 1995 | 6.7624062787606 | | 1994 | 6.6705098589244 | | 1993 | 6.7656321565533 | | 1992 | 6.8005321512846 | | 1991 | 6.8631283933416 | | 1990 | 6.8779538252774 | | 1989 | 6.8105973235047 | | 1988 | 6.9293606219639 | | 1987 | 6.9874186151264 | | 1986 | 7.112602482993 | | 1985 | 7.1105411147552 | | 1984 | 7.1856722327766 | | 1983 | 7.2951488137792 | | 1982 | 7.1252009646594 | | 1981 | 7.0268905905654 | | 1980 | 7.0862396921289 | | 1979 | 7.0242396251907 | | 1978 | 7.127527493102 | | 1977 | 7.5966719859618 | | 1976 | 8.1614943431571 | | 1975 | 8.3085017647158 | | 1974 | 8.1894730813304 | | 1973 | 8.0624213168992 | | 1972 | 8.5036163217712 | | 1971 | 8.34613268283 | | 1970 | 8.6559776844311 | | 1969 | 8.9585011239606 | | 1968 | 9.1994646327718 | | 1967 | 9.3348358601048 | | 1966 | 9.8014641649044 | | 1965 | 10.674970511684 | | 1964 | 11.561034793774 | | 1963 | 10.770002158017 | | 1962 | 10.856995457147 | | 1961 | 13.588513834929 | | 1960 | 20.886353179902 |
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