Year % 2007 19.8 2008 19.79 2009 19.74 2010 19.35 2011 19.41 2012 18.96 2013 19.11 2014 18.8 2015 19.09 2016 18.81 2017 20.48 2018 21.01 2019 21.7 2020 19.95 2021 19.95