Year	Percent
1970	51.78571
1971	52.32558
1972	52.55682
1973	53.05556
1974	53.38753
1975	53.84615
1976	54.14508
1977	54.82234
1978	55.08685
1979	55.58252
1980	56.05701
1981	56.38051
1982	56.81818
1983	57.11111
1984	57.6087
1985	58.08511
1986	58.54167
1987	58.97959
1988	59.4
1989	59.88258
1990	60.46065
1991	60.90226
1992	61.32597
1993	61.73285
1994	62.30088
1995	62.56499
1996	63.09524
1997	63.77295
1998	64.15712
1999	64.687
2000	65.19685
2001	65.68779
2002	66.16085
2003	66.66667
2004	67.10526
2005	67.57532
2006	68.26516
2007	68.69806
2008	69.2517
2009	69.7861
2010	70.30223
2011	70.80103
2012	71.3198
2013	71.78527
2014	72.26994
2015	72.94686