| Year | Value | | 2021 | 8.2062922970999 | | 2020 | 8.7096448269135 | | 2019 | 8.3184580447599 | | 2018 | 8.6398307197092 | | 2017 | 7.0812442491976 | | 2016 | 6.8075889370106 | | 2015 | 5.4489913851297 | | 2014 | 4.0985006959006 | | 2012 | 8.1723772805196 | | 2011 | 7.5404036241749 | | 2010 | 8.4078921966114 | | 2009 | 7.9085033836688 | | 2008 | 8.2843508895808 | | 2004 | 6.2119799571474 |
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