| Year | Value | | 2022 | 50.572442536788 | | 2021 | 50.580794437154 | | 2020 | 50.588557914349 | | 2019 | 50.598879481311 | | 2018 | 50.611614619941 | | 2017 | 50.625718586094 | | 2016 | 50.64209785947 | | 2015 | 50.657495500613 | | 2014 | 50.671631659545 | | 2013 | 50.686262233812 | | 2012 | 50.701107482794 | | 2011 | 50.716821403719 | | 2010 | 50.733761952322 | | 2009 | 50.751441486135 | | 2008 | 50.770992464245 | | 2007 | 50.79069583261 | | 2006 | 50.809484415003 | | 2005 | 50.829190478475 | | 2004 | 50.849482915915 | | 2003 | 50.868814989256 | | 2002 | 50.879882832626 | | 2001 | 50.875298754479 | | 2000 | 50.867875209939 | | 1999 | 50.857152146467 | | 1998 | 50.843965300923 | | 1997 | 50.84635709985 | | 1996 | 50.858805493852 | | 1995 | 50.87156623733 | | 1994 | 50.851331422428 | | 1993 | 50.78715964618 | | 1992 | 50.728671672218 | | 1991 | 50.692751987927 | | 1990 | 50.650015665371 | | 1989 | 50.592298718003 | | 1988 | 50.529847351704 | | 1987 | 50.458259199479 | | 1986 | 50.39171265553 | | 1985 | 50.3340764833 | | 1984 | 50.270012524821 | | 1983 | 50.199860791119 | | 1982 | 50.130767868926 | | 1981 | 50.063007902374 | | 1980 | 49.989453841017 | | 1979 | 49.909712754273 | | 1978 | 49.824167255349 | | 1977 | 49.732098225548 | | 1976 | 49.633840573129 | | 1975 | 49.531610347244 | | 1974 | 49.42948865519 | | 1973 | 49.325563259418 | | 1972 | 49.215607069647 | | 1971 | 49.100274257174 | | 1970 | 48.979999004926 | | 1969 | 48.95295279356 | | 1968 | 49.061032823847 | | 1967 | 49.213634038455 | | 1966 | 49.364407804745 | | 1965 | 49.488336434483 | | 1964 | 49.576790801803 | | 1963 | 49.634979794912 | | 1962 | 49.673796500887 | | 1961 | 49.700946440881 | | 1960 | 49.843789520449 |
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