| Year | Value | | 2022 | 50.982093874504 | | 2021 | 50.991022288859 | | 2020 | 51.009403105211 | | 2019 | 51.035605577971 | | 2018 | 51.072240952594 | | 2017 | 51.109666233203 | | 2016 | 51.146713112521 | | 2015 | 51.186563901538 | | 2014 | 51.227962350749 | | 2013 | 51.270637544451 | | 2012 | 51.313823665987 | | 2011 | 51.359045790447 | | 2010 | 51.406982000493 | | 2009 | 51.454632045389 | | 2008 | 51.504012263605 | | 2007 | 51.559235187266 | | 2006 | 51.618183642326 | | 2005 | 51.684285305313 | | 2004 | 51.757030514747 | | 2003 | 51.832052923602 | | 2002 | 51.908053367626 | | 2001 | 51.970031800787 | | 2000 | 52.020020868658 | | 1999 | 52.070950710989 | | 1998 | 52.12351359592 | | 1997 | 52.180707168581 | | 1996 | 52.239506997221 | | 1995 | 52.299001726681 | | 1994 | 52.360100978008 | | 1993 | 52.424691588567 | | 1992 | 52.491475214813 | | 1991 | 52.548591887422 | | 1990 | 52.591178813932 | | 1989 | 52.630356313213 | | 1988 | 52.673601987435 | | 1987 | 52.723162622247 | | 1986 | 52.685961590479 | | 1985 | 52.524967420744 | | 1984 | 52.326731782778 | | 1983 | 52.121070538624 | | 1982 | 51.914791622974 | | 1981 | 51.802755881822 | | 1980 | 51.797095297292 | | 1979 | 51.804096911719 | | 1978 | 51.806923969677 | | 1977 | 51.809032064953 | | 1976 | 51.792760661709 | | 1975 | 51.744459401145 | | 1974 | 51.6894377299 | | 1973 | 51.669596956955 | | 1972 | 51.683273151793 | | 1971 | 51.695803231677 | | 1970 | 51.705178222468 | | 1969 | 51.715471639007 | | 1968 | 51.722715230829 | | 1967 | 51.725941422594 | | 1966 | 51.705570930093 | | 1965 | 51.670788741727 | | 1964 | 51.635303597668 | | 1963 | 51.593796590394 | | 1962 | 51.554138961244 | | 1961 | 51.533983906865 | | 1960 | 51.534830370142 |
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