| Year | Value | | 2020 | 17.196933186653 | | 2019 | 16.225509753065 | | 2018 | 15.257338967205 | | 2017 | 13.61000634081 | | 2016 | 14.554257368574 | | 2015 | 16.009153881649 | | 2014 | 17.725315471113 | | 2013 | 18.60002997007 | | 2012 | 16.4492037547 | | 2011 | 15.356926055708 | | 2010 | 16.055491833004 | | 2009 | 16.172302014827 | | 2008 | 17.215931901338 | | 2007 | 17.415275809946 | | 2006 | 20.062695924765 | | 2005 | 17.412182356611 | | 2004 | 20.270176843799 | | 2003 | 24.644280022766 | | 2002 | 21.061843640607 | | 2001 | 23.304628632939 |
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