| Year | Value | | 2021 | 5.9675878665686 | | 2020 | 5.6785350767888 | | 2019 | 5.1651537280005 | | 2018 | 5.1470931104124 | | 2017 | 5.1539878275393 | | 2016 | 5.2993491608457 | | 2015 | 5.3694212913521 | | 2014 | 5.4304706807865 | | 2013 | 5.4986203400781 | | 2012 | 5.4376689530081 | | 2011 | 5.3208277762313 | | 2010 | 5.35876485852 | | 2009 | 5.3900558659687 | | 2008 | 5.4579310778618 | | 2007 | 5.4946202727771 | | 2006 | 5.5592027689956 | | 2005 | 5.5708711745039 | | 2004 | 5.6154946642215 | | 2003 | 5.7411345879715 | | 2002 | 5.6065879300382 | | 2001 | 5.6890107662426 | | 2000 | 5.7715321229136 | | 1999 | 5.8690799159031 | | 1998 | 6.0035482458961 | | 1997 | 6.0420011455781 | | 1996 | 6.1237308773172 | | 1995 | 6.2901594693061 | | 1994 | 6.4422024330211 | | 1993 | 6.5405228268976 | | 1992 | 6.7004073729856 | | 1991 | 6.9717626502043 | | 1990 | 7.5671345944957 | | 1989 | 7.6332824340284 | | 1988 | 8.0750172638014 | | 1987 | 8.3031470947741 | | 1986 | 8.7179682958507 | | 1985 | 9.0809445718506 | | 1984 | 9.5566946605998 | | 1983 | 10.261946545409 | | 1982 | 11.149252341041 | | 1981 | 11.488340926614 | | 1980 | 11.607041673813 | | 1979 | 11.948885339919 | | 1978 | 12.669307550816 | | 1977 | 13.042996034917 | | 1976 | 13.970495791995 | | 1975 | 14.102088637944 | | 1974 | 14.557088647466 | | 1973 | 15.136368686497 | | 1972 | 15.635539756127 | | 1971 | 16.118125584157 | | 1970 | 16.659190142336 | | 1969 | 17.1565966295 | | 1968 | 17.692814918481 | | 1967 | 18.099394431586 | | 1966 | 18.551736867205 | | 1965 | 19.013227813098 | | 1964 | 19.534326952011 | | 1963 | 20.108028385418 | | 1962 | 21.013653624961 | | 1961 | 21.21702247731 | | 1960 | 21.880451552308 |
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