Year	Percent
1981	43.1
1988	40
1990	36.2
1992	29.1
1994	19.5
1996	15.2
1998	14.6
1999	18.4
2000	18.6
2002	12.1
2004	9
2006	6.2
2007	4.4
2008	3.4
2009	3
2010	2.5
2011	1.4
2012	1.5
2013	1.1
2014	0.9
2015	0.5