| Year | Value | | 2021 | 6.7575944716205 | | 2020 | 6.4928597088458 | | 2019 | 6.4623522588099 | | 2018 | 6.4177777036809 | | 2017 | 6.4169048727933 | | 2016 | 6.5140194567205 | | 2015 | 6.601148752625 | | 2014 | 6.485718320897 | | 2013 | 6.4447740111783 | | 2012 | 6.3790703140339 | | 2011 | 6.3508404118277 | | 2010 | 6.3695792636746 | | 2009 | 6.4735831608563 | | 2008 | 6.3943573572533 | | 2007 | 6.3785936310408 | | 2006 | 6.2876798712458 | | 2005 | 6.2777261586652 | | 2004 | 6.260822649501 | | 2003 | 6.2592878357169 | | 2002 | 6.335196480344 | | 2001 | 6.152791470565 | | 2000 | 6.1241406832731 | | 1999 | 6.0411966639769 | | 1998 | 6.0219395085181 | | 1997 | 6.052505216666 | | 1996 | 6.0956867733711 | | 1995 | 6.1399669001615 | | 1994 | 6.2032533865667 | | 1993 | 6.2556857428436 | | 1992 | 6.3100395316266 | | 1991 | 6.3799759421518 | | 1990 | 6.4707149956397 | | 1989 | 6.5589454468293 | | 1988 | 6.6311172150441 | | 1987 | 6.7861482560081 | | 1986 | 6.8700256514047 | | 1985 | 6.8943621843796 | | 1984 | 7.0647081909553 | | 1983 | 7.2509353949012 | | 1982 | 7.4302895371711 | | 1981 | 7.522691389768 | | 1980 | 7.6125430378825 | | 1979 | 7.5513060153357 | | 1978 | 7.7071383629502 | | 1977 | 7.9347368857771 | | 1976 | 8.0607508292006 | | 1975 | 8.3621995861792 | | 1974 | 8.3599239042011 | | 1973 | 8.6676583164237 | | 1972 | 8.6995963692663 | | 1971 | 8.8944338770815 | | 1970 | 9.1180580405656 | | 1969 | 9.4043547710132 | | 1968 | 9.6202968050832 | | 1967 | 9.9601492395916 | | 1966 | 10.236300975706 | | 1965 | 10.573773114903 | | 1964 | 11.301136094017 | | 1963 | 11.532402753269 | | 1962 | 12.035524733088 | | 1961 | 12.535804713717 | | 1960 | 13.034974528257 |
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