Year	Percent
1960	30.51
1961	31.797
1962	33.214
1963	34.662
1964	36.141
1965	37.643
1966	38.84
1967	39.004
1968	39.169
1969	39.334
1970	39.5
1971	39.665
1972	39.831
1973	39.997
1974	40.163
1975	40.33
1976	40.497
1977	40.928
1978	41.794
1979	42.665
1980	43.542
1981	44.42
1982	45.303
1983	46.189
1984	47.079
1985	47.968
1986	48.86
1987	49.722
1988	50.511
1989	51.298
1990	52.085
1991	52.871
1992	53.657
1993	54.439
1994	55.219
1995	55.997
1996	56.774
1997	57.544
1998	58.313
1999	59.118
2000	59.919
2001	60.712
2002	61.501
2003	62.284
2004	63.061
2005	63.83
2006	64.593
2007	65.348
2008	66.097
2009	66.826
2010	67.54
2011	68.236
2012	68.915
2013	69.576
2014	70.221
2015	70.848
2016	71.459
2017	72.052