Year	Percent
1979	36
1989	42.9
1991	42.5
1995	43.2
1997	44.1
1998	43.4
1999	42.5
2000	43.4
2001	42.8
2002	43.1
2003	42.3
2004	41.4
2005	40.3
2006	41.2
2007	39.9
2008	41.1
2009	40.2
2010	39.8
2011	39.7
2012	39.8
2013	39.6
2014	38.6
2015	39
2016	37.9