| Year | Value | | 2022 | 43.875265769746 | | 2021 | 37.620363950489 | | 2020 | 32.95375826232 | | 2019 | 37.614446203308 | | 2018 | 40.150109308268 | | 2017 | 38.726068771175 | | 2016 | 37.383324514414 | | 2015 | 36.487922092945 | | 2014 | 43.29927206182 | | 2013 | 45.556495264189 | | 2012 | 47.779012409189 | | 2011 | 48.527072629867 | | 2010 | 49.829153307691 | | 2009 | 47.205596692207 | | 2008 | 57.413788344189 | | 2007 | 56.941871874839 | | 2006 | 54.571698905799 | | 2005 | 51.856866248805 | | 1991 | 32.259322472199 | | 1990 | 23.977688119031 | | 1989 | 17.491850008917 | | 1988 | 16.975973348139 | | 1987 | 21.346927077528 | | 1986 | 21.779972003315 | | 1985 | 29.398906614067 | | 1984 | 30.617712836669 | | 1983 | 36.525028630643 | | 1982 | 35.537090110118 | | 1981 | 36.886627295704 | | 1980 | 39.374769410188 | | 1979 | 36.662099826787 | | 1978 | 33.980837148471 | | 1977 | 31.520255831838 |
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