| Year | Value | | 2019 | 13.583633731759 | | 2018 | 12.859589627012 | | 2017 | 13.05865998156 | | 2016 | 13.867382115638 | | 2015 | 13.768718607756 | | 2014 | 13.166400070041 | | 2013 | 12.100076324841 | | 2012 | 11.361662034537 | | 2011 | 11.40523856727 | | 2010 | 10.507793254332 | | 2009 | 10.316217859631 | | 2008 | 10.341911374492 | | 2007 | 10.883524476827 | | 2006 | 13.885915624064 | | 2005 | 12.877743796943 | | 2004 | 13.14635208176 |
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