Year	Percent
1970	51.9833
1971	52.44898
1972	52.98805
1973	53.50195
1974	53.9924
1975	54.54545
1976	55.17241
1977	55.85106
1978	56.22837
1979	57.022
1980	57.52066
1981	58.15832
1982	58.76777
1983	59.50541
1984	60.2118
1985	60.94675
1986	61.64978
1987	62.32295
1988	63.1068
1989	63.8587
1990	64.62766
1991	65.4498
1992	66.28352
1993	67.16792
1994	68.05897
1995	68.99879
1996	69.86951
1997	70.71179
1998	71.41217
1999	72.20339
2000	72.96997
2001	73.79386
2002	74.40605
2003	75.18637
2004	75.86569
2005	76.60455
2006	77.14286
2007	77.84491
2008	78.52883
2009	79.01961
2010	79.67086
2011	80.30593
2012	80.75472
2013	81.36067
2014	81.86004
2015	82.43858