| Year | Value | | 2021 | 8.0020002930201 | | 2020 | 7.9126970396102 | | 2019 | 8.9481293430532 | | 2018 | 8.6547106117996 | | 2017 | 8.7678025955749 | | 2016 | 8.6386650791029 | | 2015 | 9.503163019629 | | 2014 | 10.041209460892 | | 2013 | 11.565994253403 | | 2012 | 8.8318434346312 | | 2011 | 8.8750786080703 | | 2010 | 9.7980275554948 | | 2009 | 10.509534352087 |
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