| Year | Value | | 2020 | 6.4263149509804 | | 2019 | 18.860592307692 | | 2018 | 25.196273850099 | | 2017 | 10.407880669862 | | 2016 | 6.0034914881246 | | 2015 | 5.4182436785503 | | 2014 | 12.544066611708 | | 2013 | 5.4665415567515 | | 2012 | 4.8918015564309 | | 2011 | 4.9640731046676 | | 2010 | 4.6611358838246 | | 2009 | 4.3960160494012 | | 2008 | 4.2880163833566 |
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