| Year | Value | | 2022 | 3.0210780434718 | | 2021 | 2.626143856211 | | 2020 | 2.2798794111475 | | 2019 | 3.3497137383055 | | 2018 | 3.1954011401659 | | 2017 | 2.0613444319728 | | 2016 | 4.013856540873 | | 2015 | 8.9085352033416 | | 2014 | 8.4606168602683 | | 2013 | 6.5698143952479 | | 2012 | 8.7711241134394 | | 2011 | 6.0944153222839 | | 2010 | 4.1406325429994 | | 2009 | 4.9684626329985 | | 2008 | 5.7612890330715 |
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