Year % 2004 17 2005 15.4 2006 14 2007 12.9 2008 12.4 2009 12.9 2010 12.4 2011 13 2012 13.3 2013 13.4 2014 13.3 2015 12.9 2016 9.9 2017 9.4 2018 9.5 2019 8.8 2020 9.2 2021 8.7