| Year | Value | | 2021 | 7.642363010179 | | 2020 | 7.0015367440544 | | 2019 | 7.6386381785019 | | 2018 | 7.7317730453351 | | 2017 | 6.9971423313948 | | 2016 | 7.3169118475576 | | 2015 | 8.498348828208 | | 2014 | 8.6353777373447 | | 2013 | 8.9624133154182 | | 2012 | 9.024562690195 | | 2011 | 8.6859247055782 | | 2010 | 7.8346847494673 | | 2009 | 7.4981162627693 | | 2008 | 7.6560298828247 | | 2007 | 6.9172626685682 | | 2006 | 7.0433040578091 | | 2005 | 7.1356700394615 | | 2004 | 7.0464484181855 | | 2003 | 6.9652139037587 | | 2002 | 6.6913841906781 | | 2001 | 6.6111602714382 |
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