| Year | Value | | 2021 | 9.6564736523872 | | 2020 | 9.6299070106755 | | 2019 | 9.6021641378621 | | 2018 | 9.5504859291922 | | 2017 | 9.485023403376 | | 2016 | 9.401661013642 | | 2015 | 9.2996465769267 | | 2014 | 9.241335926865 | | 2013 | 9.1675608214907 | | 2012 | 9.0333371890776 | | 2011 | 8.8499714270682 | | 2010 | 8.7013128609715 | | 2009 | 8.6130153924156 | | 2008 | 8.3893633764055 | | 2007 | 8.2624856320467 | | 2006 | 8.1406387916961 | | 2005 | 8.0872524640214 | | 2004 | 7.9438212863097 | | 2003 | 7.9225181604522 | | 2002 | 7.7027713433715 | | 2001 | 7.6290622490858 | | 2000 | 7.5722915202615 | | 1999 | 7.558096192759 | | 1998 | 7.4522734750845 | | 1997 | 7.4227504638425 | | 1996 | 7.372448548614 | | 1995 | 7.2672294750932 | | 1994 | 7.063650680128 | | 1993 | 6.9852762314008 | | 1992 | 6.8474540646763 | | 1991 | 6.732157671438 | | 1990 | 6.6365078174808 | | 1989 | 6.5689304970223 | | 1988 | 6.5316698190753 | | 1987 | 6.4817913206416 | | 1986 | 6.4800976534622 | | 1985 | 6.4507125278995 | | 1984 | 6.345705162776 | | 1983 | 6.3202154717259 | | 1982 | 6.2897294624965 | | 1981 | 6.2560678273057 | | 1980 | 6.2569993442544 | | 1979 | 6.2398933057424 | | 1978 | 6.2245232760892 | | 1977 | 6.1578774725794 | | 1976 | 6.1438200349903 | | 1975 | 6.1107511833123 | | 1974 | 6.0586106465249 | | 1973 | 6.0703633059788 | | 1972 | 6.0580846348478 | | 1971 | 6.0236196751902 | | 1970 | 5.9784002932316 | | 1969 | 5.9607020707047 | | 1968 | 5.9190392693496 | | 1967 | 5.8475689353177 | | 1966 | 5.8169569379806 | | 1965 | 5.7885042931527 | | 1964 | 5.7411678989301 | | 1963 | 5.6997591390467 | | 1962 | 5.6164335363365 | | 1961 | 5.5737969093399 |
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