Year	Percent
1960	16.073
1961	16.805
1962	17.969
1963	19.194
1964	20.484
1965	21.833
1966	23.247
1967	24.724
1968	26.264
1969	27.861
1970	29.518
1971	30.205
1972	30.561
1973	30.917
1974	31.277
1975	31.639
1976	32.004
1977	32.37
1978	32.738
1979	33.109
1980	33.482
1981	33.857
1982	34.826
1983	35.887
1984	36.963
1985	38.049
1986	39.149
1987	40.26
1988	41.382
1989	42.511
1990	43.648
1991	44.793
1992	45.764
1993	46.717
1994	47.673
1995	48.631
1996	49.592
1997	50.55
1998	51.509
1999	52.467
2000	53.424
2001	54.377
2002	55.555
2003	56.766
2004	57.972
2005	59.165
2006	60.348
2007	61.52
2008	62.68
2009	63.823
2010	64.952
2011	66.064
2012	67.16
2013	68.208
2014	69.213
2015	70.174
2016	71.092
2017	71.968