Year % 2004 35.9 2005 38.3 2006 35.5 2007 34.5 2008 35.6 2009 36.1 2010 36.1 2011 35.4 2012 34.9 2013 36.5 2014 34.5 2015 35 2016 34 2017 32.6 2018 33.8 2019 32.8 2020 33.3 2021 33.3