Year | Value | 2020 | 18.080951138033 | 2019 | 18.366750773919 | 2018 | 20.954220876249 | 2017 | 19.929109243697 | 2016 | 19.815381642882 | 2015 | 20.282907415144 | 2014 | 19.337333363492 | 2013 | 18.390078012927 | 2012 | 18.069180247744 | 2011 | 17.450908081299 | 2010 | 16.598116708666 | 2009 | 15.420823380205 | 2008 | 16.356768886642 |
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