Year	Percent
1980	30.2
1986	32.4
1987	34.4
1991	36.6
1992	34.3
1993	33.2
1994	34.4
1995	37
1996	36.9
1997	36.4
1998	38
1999	37
2000	37.7
2001	39.5
2002	40.5
2003	37.1
2004	34.7
2005	34.6
2006	33.6
2007	33.4
2008	31.6
2009	31.1
2010	31
2011	30.6
2012	29.4
2013	29.3
2014	29.5
2016	30.9