Year	Percent
1983	53.08
1984	54.01
1985	53.84
1986	54.73
1987	55.21
1988	55.02
1989	55.99
1990	56.21
1991	55.5
1992	57.27
1993	58.63
1994	59.83
1995	60.83
1996	61.8
1997	62.38
1998	62.83
1999	63.3
2000	63.83
2001	64.6
2002	65.15
2003	66.2
2004	66.39
2005	67.79
2006	68.08
2007	67.85
2008	68.87
2009	69.52
2010	70.04
2011	70.12
2012	70.25
2013	70.78
2014	70.52
2015	70.92
2016	71.31
2017	71.32