Year	Percent
1981	19.6
1988	14.3
1990	9.4
1992	6.6
1994	3.2
1996	2.2
1998	1.5
1999	2.5
2000	2.5
2002	1.1
2004	0.8
2006	0.7
2007	0.3
2008	0.1
2009	0.2
2010	0.1
2011	0
2012	0.1
2013	0
2014	0
2015	0