| Year | Value | | 2021 | 45.361652421652 | | 2020 | 44.115379136533 | | 2019 | 47.304404996713 | | 2018 | 47.381108919571 | | 2017 | 47.370151216305 | | 2016 | 47.370151216305 | | 2015 | 47.381108919571 | | 2014 | 47.35919351304 | | 2013 | 47.674803932398 | | 2012 | 48.569534960786 | | 2011 | 48.547442836629 | | 2010 | 48.514304650392 | | 2009 | 50.646194631614 | | 2008 | 51.16616449057 | | 2007 | 51.244280772235 | | 2006 | 51.300078116282 | | 2005 | 51.344715991519 | | 2004 | 51.305512162464 | | 2003 | 51.294353938853 | | 2002 | 51.39477795135 | | 2001 | 51.39477795135 | | 2000 | 51.350145056907 | | 1999 | 53.619153674833 | | 1998 | 53.663697104677 | | 1997 | 53.674832962138 | | 1996 | 53.538838192744 | | 1995 | 53.483196082796 | | 1994 | 52.848876029379 | | 1993 | 52.837747607389 | | 1992 | 52.765105151886 | | 1991 | 56.259040836764 | | 1990 | 56.236786469345 | | 1989 | 56.214532101925 | | 1988 | 56.192277734505 | | 1987 | 56.163773920783 | | 1986 | 56.108144192256 | | 1985 | 56.030262572319 | | 1984 | 55.93012906097 | | 1983 | 55.852247441033 | | 1982 | 55.83490933363 | | 1981 | 55.962180200222 | | 1980 | 55.911467022578 | | 1979 | 56.016459074733 | | 1978 | 56.085361787262 | | 1977 | 56.369497554469 | | 1976 | 56.787103946637 | | 1975 | 56.996776703346 | | 1974 | 57.007891519395 | | 1973 | 57.530287873736 | | 1972 | 57.541402689785 | | 1971 | 57.619206402134 | | 1970 | 57.712825072238 | | 1969 | 57.784198244249 | | 1968 | 57.861984664963 | | 1967 | 57.944444444444 | | 1966 | 58.049105654927 | | 1965 | 58.087091757387 | | 1964 | 57.902921248473 | | 1963 | 57.702987892925 | | 1962 | 57.676071984003 | | 1961 | 57.685473123056 |
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