| Year | Value | | 2021 | 41.898744483494 | | 2020 | 41.862820555674 | | 2019 | 41.769922615579 | | 2018 | 41.73020024921 | | 2017 | 41.600579377395 | | 2016 | 41.475934480516 | | 2015 | 41.350559809132 | | 2014 | 41.229929916094 | | 2013 | 41.107635440701 | | 2012 | 40.918691831043 | | 2011 | 40.882990705645 | | 2010 | 40.703154903098 | | 2009 | 40.532972009947 | | 2008 | 40.346000965955 | | 2007 | 40.262076561358 | | 2006 | 40.103370259359 | | 2005 | 39.952385287228 | | 2004 | 39.60319098837 | | 2003 | 39.775539515813 | | 2002 | 39.524181088308 | | 2001 | 39.432455817898 | | 2000 | 39.243849058319 | | 1999 | 39.134970312313 | | 1998 | 38.934465901622 | | 1997 | 38.83748174668 | | 1996 | 38.642761751337 | | 1995 | 38.482892424168 | | 1994 | 38.194459799477 | | 1993 | 37.936101586559 | | 1992 | 38.423098168037 | | 1991 | 37.703377416641 | | 1990 | 37.580426681208 | | 1989 | 37.528675027774 | | 1988 | 37.377248382071 | | 1987 | 37.208106888068 | | 1986 | 37.04832711339 | | 1985 | 36.913739755345 | | 1984 | 36.7074573715 | | 1983 | 36.61685986282 | | 1982 | 36.652296136484 | | 1981 | 36.528984786021 | | 1980 | 36.507925627921 | | 1979 | 36.411512906725 | | 1978 | 36.396673276162 | | 1977 | 36.300293810314 | | 1976 | 36.271431144883 | | 1975 | 36.227531021349 | | 1974 | 36.158496709881 | | 1973 | 36.172446053939 | | 1972 | 36.1076666295 | | 1971 | 36.020238103256 | | 1970 | 35.987249736363 | | 1969 | 35.947401784673 | | 1968 | 35.960081623239 | | 1967 | 35.907075740708 | | 1966 | 35.87664412815 | | 1965 | 35.85377884549 | | 1964 | 35.776660483063 | | 1963 | 35.698419643124 | | 1962 | 35.591326973792 | | 1961 | 35.522564832846 |
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