| Year | Value | | 2021 | 67.167941585714 | | 2020 | 67.47043980976 | | 2019 | 68.024895134077 | | 2018 | 67.590202338968 | | 2017 | 67.193842520317 | | 2016 | 66.778289295964 | | 2015 | 66.224329793236 | | 2014 | 65.831107903747 | | 2013 | 65.517816287482 | | 2012 | 64.965663791655 | | 2011 | 64.407399339935 | | 2010 | 63.674045230023 | | 2009 | 63.163685267736 | | 2008 | 62.196210974358 | | 2007 | 62.094290429591 | | 2006 | 61.639155723672 | | 2005 | 61.066316905605 | | 2004 | 60.352092678032 | | 2003 | 60.083078279316 | | 2002 | 59.510812075754 | | 2001 | 58.989386043639 | | 2000 | 58.514756893143 | | 1999 | 58.195417123839 | | 1998 | 57.106502496685 | | 1997 | 56.711293980414 | | 1996 | 55.966778618583 | | 1995 | 55.889676927781 | | 1994 | 55.416949931889 | | 1993 | 54.947626897253 | | 1992 | 54.500617004932 | | 1991 | 53.820771336703 | | 1990 | 54.138181417118 | | 1989 | 53.845310314884 | | 1988 | 53.121382618933 | | 1987 | 53.247000674845 | | 1986 | 52.606698961669 | | 1985 | 51.966853410473 | | 1984 | 51.609995735818 | | 1983 | 51.336763090732 | | 1982 | 51.845852179801 | | 1981 | 51.539376136729 | | 1980 | 51.229855129198 | | 1979 | 50.769977922889 | | 1978 | 50.257991508775 | | 1977 | 49.746916671209 | | 1976 | 48.986586769581 | | 1975 | 48.434474501901 | | 1974 | 48.478885743825 | | 1973 | 48.538273285584 | | 1972 | 47.984004385273 | | 1971 | 45.072046746635 | | 1970 | 46.238163837955 | | 1969 | 47.338590934474 | | 1968 | 47.070463682354 | | 1967 | 46.710702467659 | | 1966 | 46.074829001161 | | 1965 | 45.705982538323 | | 1964 | 45.686119410039 | | 1963 | 44.967026332612 | | 1962 | 44.789323372093 | | 1961 | 44.324372914862 | | 1960 | 43.825901849107 |
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