| Year | Value | | 2022 | 50.345181168208 | | 2021 | 50.357996147461 | | 2020 | 50.377149620778 | | 2019 | 50.393048593356 | | 2018 | 50.39821083356 | | 2017 | 50.392064621814 | | 2016 | 50.372832760663 | | 2015 | 50.337364415701 | | 2014 | 50.294915814212 | | 2013 | 50.257435796167 | | 2012 | 50.236719007549 | | 2011 | 50.228169801057 | | 2010 | 50.219349795596 | | 2009 | 50.222576244783 | | 2008 | 50.252752357848 | | 2007 | 50.29578757023 | | 2006 | 50.329647242419 | | 2005 | 50.349471743402 | | 2004 | 50.36210186812 | | 2003 | 50.370919862476 | | 2002 | 50.377616497647 | | 2001 | 50.382543396524 | | 2000 | 50.375042795067 | | 1999 | 50.356052255393 | | 1998 | 50.332951342479 | | 1997 | 50.304471693758 | | 1996 | 50.272007037178 | | 1995 | 50.245123048665 | | 1994 | 50.22651984465 | | 1993 | 50.20730214217 | | 1992 | 50.184446629189 | | 1991 | 50.157870790522 | | 1990 | 50.131977334823 | | 1989 | 50.115588951756 | | 1988 | 50.100481649929 | | 1987 | 50.078223170086 | | 1986 | 50.062367683741 | | 1985 | 50.06530601592 | | 1984 | 50.072088432243 | | 1983 | 50.073119316319 | | 1982 | 50.080275202897 | | 1981 | 50.084915980199 | | 1980 | 50.069153932574 | | 1979 | 50.035656419357 | | 1978 | 49.994887088952 | | 1977 | 49.948148307325 | | 1976 | 49.899181312078 | | 1975 | 49.851571853495 | | 1974 | 49.812774202845 | | 1973 | 49.787994332702 | | 1972 | 49.769178277676 | | 1971 | 49.746148827205 | | 1970 | 49.718087303298 | | 1969 | 49.698480211151 | | 1968 | 49.687651270921 | | 1967 | 49.673336940718 | | 1966 | 49.652560776525 | | 1965 | 49.631909736411 | | 1964 | 49.613851869932 | | 1963 | 49.589328936672 | | 1962 | 49.542897967791 | | 1961 | 49.491287688924 | | 1960 | 49.479397902932 |
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