Year	Percent
1960	33.735
1961	34.389
1962	35.051
1963	35.718
1964	36.391
1965	37.068
1966	37.752
1967	38.557
1968	39.437
1969	40.321
1970	41.212
1971	42.11
1972	43.014
1973	43.92
1974	44.832
1975	45.747
1976	46.666
1977	47.47
1978	48.21
1979	48.951
1980	49.693
1981	50.434
1982	51.175
1983	51.916
1984	52.657
1985	53.395
1986	54.133
1987	54.717
1988	55.257
1989	55.794
1990	56.33
1991	56.865
1992	57.653
1993	58.519
1994	59.38
1995	60.236
1996	61.087
1997	61.853
1998	62.588
1999	63.318
2000	64.042
2001	64.758
2002	65.468
2003	66.172
2004	66.869
2005	67.558
2006	68.239
2007	68.864
2008	69.458
2009	70.045
2010	70.626
2011	71.2
2012	71.755
2013	72.295
2014	72.83
2015	73.358
2016	73.88
2017	74.394