Year	Percent
1987	35.1
1994	32
2002	31.9
2003	32.4
2004	30.8
2005	32.8
2006	29.6
2007	28.2
2008	29.4
2009	29.2
2010	29.3
2011	30.5
2012	30.5
2013	30.6
2014	31.6
2015	33.5
2016	32.1