Year	Percent
1987	32.4
1989	35.6
1993	37.9
2004	38.4
2005	42.2
2006	37.1
2007	36.7
2008	36.4
2009	35.9
2010	37.7
2011	37.6
2012	38.6
2013	39.3
2014	38.7
2015	38.4