Year % 1975 34.2 1981 32.8 1987 33.2 1992 34.5 1995 34.4 2000 36.5 2001 36.3 2002 35.8 2003 34.7 2004 35.5 2005 35.6 2006 35.6 2007 36.2 2008 36.9 2009 36.2 2010 36.6 2011 36.3 2012 36.4 2013 37.1 2014 37 2015 37.6 2016 37.8 2017 37.1 2018 37.8 2019 37.5 2020 37.2 2021 37.7