| Year | Value | | 2021 | 8.7998266897747 | | 2020 | 8.7998266897747 | | 2019 | 9.1278162911612 | | 2018 | 9.4575389948007 | | 2017 | 9.7872616984402 | | 2016 | 10.11698440208 | | 2015 | 10.11698440208 | | 2014 | 10.11698440208 | | 2013 | 10.11698440208 | | 2012 | 9.9967504332756 | | 2011 | 9.8635181975737 | | 2010 | 9.6187175043328 | | 2009 | 9.3934142114385 | | 2008 | 9.1161178509532 | | 2007 | 8.9384748700173 | | 2006 | 8.7738301559792 | | 2005 | 8.6005199306759 | | 2004 | 8.4315424610052 | | 2003 | 8.2798960138648 | | 2002 | 8.1282495667244 | | 2001 | 7.9766031195841 | | 2000 | 7.8249566724437 | | 1999 | 7.6819757365685 | | 1998 | 7.6473136915078 | | 1997 | 7.6083188908146 | | 1996 | 7.3656845753899 | | 1995 | 7.3656845753899 | | 1994 | 7.3223570190641 | | 1993 | 7.2140381282496 | | 1992 | 7.209705372617 | | 1991 | 7.2010398613518 | | 1990 | 7.1923743500867 | | 1989 | 7.157712305026 | | 1988 | 7.1490467937608 | | 1987 | 7.105719237435 | | 1986 | 7.0970537261698 | | 1985 | 7.0840554592721 | | 1984 | 7.0623916811092 | | 1983 | 7.0190641247834 | | 1982 | 7.0060658578856 | | 1981 | 6.971403812825 | | 1980 | 6.9584055459272 | | 1979 | 6.9540727902946 | | 1978 | 6.9237435008665 | | 1977 | 6.8890814558059 | | 1976 | 6.8457538994801 | | 1975 | 6.7937608318891 | | 1974 | 6.7374350086655 | | 1973 | 6.6377816291161 | | 1972 | 6.5511265164645 | | 1971 | 6.421143847487 | | 1970 | 6.3994800693241 | | 1969 | 6.3778162911612 | | 1968 | 6.369150779896 | | 1967 | 6.3258232235702 | | 1966 | 6.3258232235702 | | 1965 | 6.2824956672444 | | 1964 | 6.2824956672444 | | 1963 | 6.4991334488735 | | 1962 | 6.7157712305026 | | 1961 | 6.7157712305026 |
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