Year	Percent
1960	35.584
1961	36.959
1962	38.358
1963	39.776
1964	41.214
1965	42.663
1966	44.126
1967	45.6
1968	48.229
1969	50.861
1970	53.492
1971	56.103
1972	58.685
1973	61.212
1974	61.093
1975	60.632
1976	60.168
1977	59.704
1978	59.237
1979	58.769
1980	58.298
1981	58.826
1982	59.614
1983	60.396
1984	61.175
1985	61.945
1986	62.711
1987	63.47
1988	64.224
1989	64.714
1990	65.054
1991	65.392
1992	65.728
1993	66.063
1994	66.396
1995	66.727
1996	67.058
1997	67.385
1998	67.712
1999	68.053
2000	68.58
2001	69.101
2002	69.618
2003	70.13
2004	70.637
2005	71.138
2006	71.635
2007	72.126
2008	72.613
2009	73.093
2010	73.568
2011	74.038
2012	74.498
2013	74.946
2014	75.384
2015	75.811
2016	76.228
2017	76.634