Year	Percent
1977	46.64
1978	47.34
1979	48.2
1980	48.73
1981	49.92
1982	51.38
1983	52.02
1984	53.68
1985	55.5
1986	56.37
1987	57.31
1988	57.99
1989	58.84
1990	58.6
1991	59.33
1992	57.36
1993	58.8
1994	59.34
1995	59.76
1996	60.52
1997	61.5
1998	61.5
1999	62.2
2000	62.98
2001	63.06
2002	63.48
2003	63.52
2004	65.03
2005	65.12
2006	65.68
2007	65.9
2008	66.5
2009	67.07
2010	67.62
2011	68.03
2012	68.72
2013	69.35
2014	69.45
2015	69.65
2016	69.99
2017	70.21