Year Percent 2005 18.4251 2006 18.08266 2007 19.07483 2008 18.52395 2009 17.12958 2010 16.86569 2011 16.99965 2012 16.41665 2013 15.66896 2014 16.23528 2015 16.83974 2016 16.8355