Year	Percent
1988	40.7
1993	36.9
1994	40.6
1998	35.8
1999	31.1
2000	26.7
2002	23.4
2003	24.6
2004	23.8
2005	22.7
2006	22.1
2007	22.6
2008	20.2
2010	17.5
2011	12
2013	15.3
2014	14.5
2015	14.1