Year	Percent
1980	10.2
1986	9.8
1987	9.2
1991	9.1
1992	9.2
1993	9.3
1994	9
1995	8.4
1996	8.2
1997	8.2
1998	7.8
1999	8
2000	7.6
2001	7.1
2002	6.9
2003	7.6
2004	8.1
2005	8.4
2006	8.6
2007	8.7
2008	9.2
2009	9.3
2010	9.5
2011	9.7
2012	9.9
2013	10
2014	9.8
2016	9.6