Year	Percent
1987	15.4
1989	14.2
1993	13.8
2004	13.5
2005	12.5
2006	13.8
2007	13.8
2008	13.9
2009	14.1
2010	13.4
2011	13.7
2012	13.3
2013	13
2014	13.2
2015	13.3