Year	Percent
1981	69.6
1988	68.5
1990	64.7
1992	58
1994	50.9
1996	45.4
1998	44.3
1999	48.3
2000	48.9
2002	40.1
2004	33.9
2006	27.1
2007	24.8
2008	23
2009	20.3
2010	17.8
2011	13.7
2012	13.7
2013	11.6
2014	10.6
2015	7.1