| Year | Value | | 2021 | 42.536137102995 | | 2020 | 42.455646107925 | | 2019 | 42.378508904316 | | 2018 | 42.301371700708 | | 2017 | 42.224234497099 | | 2016 | 42.110205587417 | | 2015 | 42.01294563504 | | 2014 | 41.915685682664 | | 2013 | 41.818425730288 | | 2012 | 41.687627863299 | | 2011 | 41.117483314888 | | 2010 | 40.580876681088 | | 2009 | 40.580876681088 | | 2008 | 40.279035449576 | | 2007 | 39.541201328101 | | 2006 | 38.83690512124 | | 2005 | 38.082302042459 | | 2004 | 38.467988060502 | | 2003 | 37.696616024416 | | 2002 | 37.344467920985 | | 2001 | 37.508803702586 | | 2000 | 37.676493275648 | | 1999 | 37.679847067109 | | 1998 | 37.847536640172 | | 1997 | 38.015226213234 | | 1996 | 37.243854177147 | | 1995 | 36.942012945635 | | 1994 | 36.942012945635 | | 1993 | 37.109702518697 | | 1992 | 37.27739209176 | | 1991 | 37.418251333132 | | 1990 | 37.361236878291 | | 1989 | 37.227085219841 | | 1988 | 37.092933561391 | | 1987 | 36.958781902941 | | 1986 | 36.824630244491 | | 1985 | 36.589864842204 | | 1984 | 36.288023610692 | | 1983 | 36.153871952242 | | 1982 | 36.019720293792 | | 1981 | 35.784954891505 | | 1980 | 35.634034275749 | | 1979 | 34.859308448201 | | 1978 | 34.007445417044 | | 1977 | 33.839755843982 | | 1976 | 32.444578596103 | | 1975 | 30.828051111782 | | 1974 | 29.719306482444 | | 1973 | 28.823391342253 | | 1972 | 28.263930798632 | | 1971 | 27.757661101053 | | 1970 | 27.861597264132 | | 1969 | 27.670488835244 | | 1968 | 27.764366659961 | | 1967 | 27.844833366861 | | 1966 | 27.258096962382 | | 1965 | 27.26480252129 | | 1964 | 27.331858110373 | | 1963 | 26.393079863207 | | 1962 | 26.124857506873 | | 1961 | 25.859987929994 |
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