| Year | Value | | 2021 | 42.638568793796 | | 2020 | 42.595286807601 | | 2019 | 42.531210303218 | | 2018 | 42.470411496036 | | 2017 | 42.326134128559 | | 2016 | 42.209870669602 | | 2015 | 42.103255441366 | | 2014 | 41.963546663519 | | 2013 | 41.823553085903 | | 2012 | 41.59925347577 | | 2011 | 41.566612586397 | | 2010 | 41.374031919934 | | 2009 | 41.183178543379 | | 2008 | 40.96787847538 | | 2007 | 40.851885969646 | | 2006 | 40.704976257631 | | 2005 | 40.569632994328 | | 2004 | 40.193584487127 | | 2003 | 40.190764343954 | | 2002 | 39.923669502143 | | 2001 | 39.810937194814 | | 2000 | 39.638460818702 | | 1999 | 39.484158040314 | | 1998 | 39.287527548124 | | 1997 | 39.139577262457 | | 1996 | 38.985974241757 | | 1995 | 38.845142324476 | | 1994 | 38.573664456767 | | 1993 | 38.311350538784 | | 1992 | 37.870723100954 | | 1991 | 37.642730250766 | | 1990 | 37.490231352257 | | 1989 | 37.40461430717 | | 1988 | 37.265373668997 | | 1987 | 37.025215961283 | | 1986 | 36.91029509098 | | 1985 | 36.681599568942 | | 1984 | 36.428833456984 | | 1983 | 36.376456252002 | | 1982 | 36.300058292689 | | 1981 | 36.252664646697 | | 1980 | 36.185835120645 | | 1979 | 36.128025836365 | | 1978 | 36.101612973719 | | 1977 | 36.019483921569 | | 1976 | 35.995861851957 | | 1975 | 35.951124442907 | | 1974 | 35.877101107569 | | 1973 | 35.873839456363 | | 1972 | 35.842533986891 | | 1971 | 35.802368102786 | | 1970 | 35.713116119621 | | 1969 | 35.685009967468 | | 1968 | 35.638664716578 | | 1967 | 35.559130286019 | | 1966 | 35.539794848013 | | 1965 | 35.514280043222 | | 1964 | 35.467685624316 | | 1963 | 35.405493287638 | | 1962 | 35.31300212027 | | 1961 | 35.263367851575 |
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