Year	Percent
1979	53.4
1989	61
1991	60.6
1995	60.5
1997	61.3
1998	60.6
1999	59.8
2000	60.3
2001	60
2002	59.9
2003	59.6
2004	58.8
2005	57.7
2006	58.5
2007	56.8
2008	57.5
2009	56.5
2010	56.1
2011	55.8
2012	56.2
2013	56
2014	54.9
2015	55.3
2016	54.8