Year Percent 1998 13.73139 2000 21.65186 2003 19.70902 2005 15.84252 2007 21.17046 2009 17.46983 2010 15.93648 2011 17.68975 2013 17.50001 2014 18.33992 2015 16.77975 2016 18.03809