| Year | Value | | 2022 | 50.274136875975 | | 2021 | 50.303851451513 | | 2020 | 50.341713912403 | | 2019 | 50.358909777818 | | 2018 | 50.347060455523 | | 2017 | 50.331337921979 | | 2016 | 50.310770378397 | | 2015 | 50.286074732335 | | 2014 | 50.253593880824 | | 2013 | 50.209754908787 | | 2012 | 50.16065436615 | | 2011 | 50.109547305622 | | 2010 | 50.054640397881 | | 2009 | 50.051405126507 | | 2008 | 50.103215548744 | | 2007 | 50.157315772294 | | 2006 | 50.213522022381 | | 2005 | 50.270887204637 | | 2004 | 50.331461665099 | | 2003 | 50.3943211691 | | 2002 | 50.455687202866 | | 2001 | 50.515114873035 | | 2000 | 50.556142525561 | | 1999 | 50.57855998075 | | 1998 | 50.600915044885 | | 1997 | 50.620633114864 | | 1996 | 50.639534883721 | | 1995 | 50.654856648785 | | 1994 | 50.663756955084 | | 1993 | 50.666144806028 | | 1992 | 50.662378677765 | | 1991 | 50.626937392069 | | 1990 | 50.616411722502 | | 1989 | 50.670695878452 | | 1988 | 50.732480250776 | | 1987 | 50.787811439935 | | 1986 | 50.837066588512 | | 1985 | 50.881690692381 | | 1984 | 50.929433199141 | | 1983 | 50.97755249819 | | 1982 | 51.020609019152 | | 1981 | 51.060068296093 | | 1980 | 51.052482895 | | 1979 | 50.990318918652 | | 1978 | 50.917348650069 | | 1977 | 50.847022021838 | | 1976 | 50.775074735336 | | 1975 | 50.698389156046 | | 1974 | 50.603327631784 | | 1973 | 50.495369458128 | | 1972 | 50.389431765477 | | 1971 | 50.289623124355 | | 1970 | 50.181547495968 | | 1969 | 50.095221972999 | | 1968 | 50.049205312432 | | 1967 | 50.004936675912 | | 1966 | 49.956506450001 | | 1965 | 49.900799328488 | | 1964 | 49.841803246473 | | 1963 | 49.782525136048 | | 1962 | 49.71098265896 | | 1961 | 49.629361007711 | | 1960 | 49.554172182532 |
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