| Year | Value | | 2022 | 51.875961632742 | | 2021 | 51.916188735791 | | 2020 | 51.947193046635 | | 2019 | 51.980275625941 | | 2018 | 52.025560981199 | | 2017 | 52.074331740415 | | 2016 | 52.129586919927 | | 2015 | 52.182240893918 | | 2014 | 52.226411808167 | | 2013 | 52.272276546184 | | 2012 | 52.313390318721 | | 2011 | 52.353373574267 | | 2010 | 52.396784981715 | | 2009 | 52.435681699393 | | 2008 | 52.463859976809 | | 2007 | 52.482672894945 | | 2006 | 52.489061950554 | | 2005 | 52.481169179231 | | 2004 | 52.468743724602 | | 2003 | 52.45468034948 | | 2002 | 52.43969882449 | | 2001 | 52.417712253911 | | 2000 | 52.388937096279 | | 1999 | 52.353169241075 | | 1998 | 52.268716798884 | | 1997 | 52.151805968383 | | 1996 | 52.048999250321 | | 1995 | 51.972956113072 | | 1994 | 51.913892697331 | | 1993 | 51.870165881052 | | 1992 | 51.85715399676 | | 1991 | 51.866805242762 | | 1990 | 51.889208251853 | | 1989 | 51.919135793711 | | 1988 | 51.873980179653 | | 1987 | 51.760798801529 | | 1986 | 51.669529133394 | | 1985 | 51.648714115036 | | 1984 | 51.689166395446 | | 1983 | 51.723623445062 | | 1982 | 51.74620318639 | | 1981 | 51.760225168574 | | 1980 | 51.771277335104 | | 1979 | 51.780478220235 | | 1978 | 51.789194861403 | | 1977 | 51.798694002837 | | 1976 | 51.802715280253 | | 1975 | 51.797021371475 | | 1974 | 51.785396442952 | | 1973 | 51.771718262375 | | 1972 | 51.75649026875 | | 1971 | 51.734740731086 | | 1970 | 51.715044998881 | | 1969 | 51.704341913683 | | 1968 | 51.698115674602 | | 1967 | 51.694388101951 | | 1966 | 51.690902881769 | | 1965 | 51.690576421839 | | 1964 | 51.694685284603 | | 1963 | 51.701610949436 | | 1962 | 51.712932106246 | | 1961 | 51.726307568743 | | 1960 | 51.736447964935 |
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