Year	Percent
1960	32.401
1961	33.522
1962	34.663
1963	35.822
1964	37
1965	38.127
1966	39.269
1967	40.422
1968	41.588
1969	42.76
1970	44.01
1971	45.349
1972	46.697
1973	48.046
1974	49.4
1975	50.614
1976	51.829
1977	53.039
1978	54.247
1979	55.402
1980	56.498
1981	57.584
1982	58.664
1983	59.736
1984	60.8
1985	61.833
1986	62.857
1987	63.87
1988	64.872
1989	65.585
1990	65.981
1991	66.374
1992	66.766
1993	67.155
1994	67.541
1995	67.926
1996	68.308
1997	68.687
1998	69.064
1999	69.482
2000	69.973
2001	70.458
2002	70.939
2003	71.415
2004	71.887
2005	72.354
2006	72.816
2007	73.273
2008	73.726
2009	74.172
2010	74.672
2011	75.188
2012	75.697
2013	76.199
2014	76.693
2015	77.181
2016	77.661
2017	78.134