| Year | Value | | 2022 | 50.166386151535 | | 2021 | 50.179381334052 | | 2020 | 50.198446973896 | | 2019 | 50.212673099226 | | 2018 | 50.222523004897 | | 2017 | 50.238331006403 | | 2016 | 50.262156650894 | | 2015 | 50.286304335945 | | 2014 | 50.308153477614 | | 2013 | 50.326098148784 | | 2012 | 50.343385066448 | | 2011 | 50.359710626935 | | 2010 | 50.365626771245 | | 2009 | 50.368363699425 | | 2008 | 50.369297367435 | | 2007 | 50.373942693518 | | 2006 | 50.3979116866 | | 2005 | 50.442064994072 | | 2004 | 50.501053214653 | | 2003 | 50.567691058679 | | 2002 | 50.635614235257 | | 2001 | 50.701948217125 | | 2000 | 50.761675107937 | | 1999 | 50.816305190924 | | 1998 | 50.869011271519 | | 1997 | 50.898030007838 | | 1996 | 50.89542528582 | | 1995 | 50.888275512754 | | 1994 | 50.883804751361 | | 1993 | 50.874368335607 | | 1992 | 50.858801142464 | | 1991 | 50.835157132936 | | 1990 | 50.81920427853 | | 1989 | 50.818977893105 | | 1988 | 50.820282742286 | | 1987 | 50.820640578846 | | 1986 | 50.822294378281 | | 1985 | 50.832484771314 | | 1984 | 50.851786163048 | | 1983 | 50.872771133994 | | 1982 | 50.886098355094 | | 1981 | 50.888790001867 | | 1980 | 50.887606994235 | | 1979 | 50.879422762191 | | 1978 | 50.861164535932 | | 1977 | 50.837791972626 | | 1976 | 50.800516446789 | | 1975 | 50.741702620404 | | 1974 | 50.668387463836 | | 1973 | 50.581795887498 | | 1972 | 50.484670600899 | | 1971 | 50.379770254457 | | 1970 | 50.274107546672 | | 1969 | 50.172007526814 | | 1968 | 50.066994103228 | | 1967 | 49.960651804961 | | 1966 | 49.849879749302 | | 1965 | 49.734728802975 | | 1964 | 49.615010388859 | | 1963 | 49.490274514966 | | 1962 | 49.361163180727 | | 1961 | 49.226503364117 | | 1960 | 49.090651373885 |
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