| Year | Value | | 2017 | 9.8984508968842 | | 2016 | 9.5026527828809 | | 2015 | 7.585382331298 | | 2014 | 6.882102942773 | | 2013 | 7.1227732902089 | | 2012 | 7.7078006028777 | | 2011 | 8.9167939959574 | | 2010 | 9.1697519191541 | | 2009 | 8.481758361036 | | 2008 | 6.0879649302077 | | 2007 | 5.2834566039828 | | 2006 | 6.9675975750702 |
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