Year	Percent
1981	42.4
1986	26.7
1989	21.8
1990	19.3
1991	23.3
1992	19.7
1993	16.1
1994	13.6
1995	14.1
1996	16.7
1997	13.8
1998	10.8
1999	13.1
2000	13.7
2001	10.6
2002	9.8
2003	9.6
2004	9.1
2005	7.7
2006	7.6
2007	5.2
2008	5.2
2009	5.5
2010	4
2011	4.3
2012	4.2
2013	4
2014	3.8
2015	4
2016	3.8