Year % 2007 3.97 2008 3.97 2009 3.97 2010 3.97 2011 3.98 2012 3.97 2013 3.04 2014 3 2015 2.96 2016 2.93 2017 2.9 2018 2.89 2019 2.88 2020 2.88 2021 2.88