| Year | Value | | 2020 | 0.017894557358482 | | 2019 | 0.018360258864226 | | 2018 | 0.020994088542333 | | 2017 | 0.019482392840875 | | 2016 | 0.019367204125288 | | 2015 | 0.019701199578357 | | 2014 | 0.019145638405826 | | 2013 | 0.018491427002108 | | 2012 | 0.018081374955314 | | 2011 | 0.017462457242646 | | 2010 | 0.016609761577959 | | 2009 | 0.015432462358472 | | 2008 | 0.016364868556441 |
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