Year Percent 2006 28.09 2007 26.04 2008 21.23 2009 21.87 2010 24.95 2011 25.4 2012 25.68 2013 25.55 2014 25.9 2015 25.48 2016 23.76 2017 19.7 2018 17.83 2019 15.88 2020 14.67