Year	Percent
1970	56.99301
1971	57.33788
1972	57.66667
1973	57.98046
1974	58.28025
1975	58.69565
1976	59.27052
1977	59.64392
1978	60
1979	60.33994
1980	60.66482
1981	61.24661
1982	61.80371
1983	61.9171
1984	62.69036
1985	63.0273
1986	63.34951
1987	63.89549
1988	64.4186
1989	64.69248
1990	65.40179
1991	65.72052
1992	66.23932
1993	66.66667
1994	67.14579
1995	67.60563
1996	68.24458
1997	68.53282
1998	69.12879
1999	69.57328
2000	70.30965
2001	70.71429
2002	71.27846
2003	71.82131
2004	72.22222
2005	72.89256
2006	73.2577
2007	73.76789
2008	74.41498
2009	74.88515
2010	75.48872
2011	76.0709
2012	76.52174
2013	76.9559
2014	77.51397
2015	78.05213