Year	Percent
1975	37.35
1978	38.55
1979	36.3
1980	41.53
1985	44.44
1986	67.35
1988	63.31
1989	62.81
1990	63.43
1991	60.95
1992	41.5
1994	46.15
1995	46.7
1996	46.96
1997	50.06
1998	50.26
1999	52.86
2000	54.12
2001	54.54
2002	55.68
2003	57.16
2004	57.16
2005	57.8
2006	58.16
2007	58.57
2008	57.85
2009	58.4
2010	57.87
2011	57.12
2012	58.05
2013	60.06
2014	60.35
2015	59.71
2016	60.22
2017	59.54