Year	Percent
1981	62.3
1982	62.7
1983	63.4
1984	62.9
1985	60
1986	62.8
1987	63.7
1988	65.5
1989	67.3
1990	64.6
1992	57.1
1993	64.3
1995	63.8
1996	63.9
1997	63.8
1998	63.8
1999	63.3
2001	62.5
2002	62.4
2003	61.8
2004	60.8
2005	60.7
2006	60.1
2007	59.3
2008	58.6
2009	58.2
2011	57.5
2012	57.2
2013	57.4
2014	56.3
2015	56.1