Year	Percent
1981	60.4
1982	60.1
1983	66.6
1984	66.7
1985	61.5
1986	42.4
1987	52.9
1988	55
1989	52.1
1990	57.8
1992	58.2
1993	56.8
1995	45
1996	45.7
1997	45.7
1998	44.6
1999	45.9
2001	41.3
2002	40.4
2003	41.7
2004	40.3
2005	38.1
2006	34.2
2007	32.1
2008	28.9
2009	27.4
2011	24
2012	20.8
2013	19.5
2014	17.9
2015	19.4