| Year | Value | | 2020 | 32.318271119843 | | 2019 | 24.889376465802 | | 2018 | 24.670642460153 | | 2017 | 24.722106733503 | | 2016 | 26.079173616836 | | 2015 | 24.991234930984 | | 2014 | 23.482022526465 | | 2013 | 21.463503206493 | | 2012 | 22.653155968492 | | 2011 | 19.557807345243 | | 2010 | 22.585825591811 | | 2009 | 22.995345187948 | | 2008 | 24.054767972904 |
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