Year	Percent
1970	54.34783
1971	55.02646
1972	55.15464
1973	55.77889
1974	56.37255
1975	56.45933
1976	56.74419
1977	57.72727
1978	57.9646
1979	58.18966
1980	58.82353
1981	59.42623
1982	60
1983	60.54688
1984	60.8365
1985	61.71004
1986	61.95652
1987	62.54417
1988	63.10345
1989	63.63636
1990	64.2623
1991	64.74359
1992	65.3125
1993	65.85366
1994	66.66667
1995	66.95652
1996	67.70538
1997	68.50829
1998	69.0027
1999	69.47368
2000	70.17995
2001	70.85427
2002	71.32353
2003	71.94245
2004	72.36534
2005	72.76888
2006	73.37808
2007	73.96061
2008	74.5182
2009	75.05241
2010	75.40984
2011	75.9519
2012	76.62083
2013	77.11538
2014	77.58945
2015	77.90055