Year	Percent
1960	31.515
1961	32.064
1962	32.598
1963	33.137
1964	33.68
1965	34.227
1966	34.949
1967	35.759
1968	36.577
1969	37.401
1970	38.234
1971	38.945
1972	39.6
1973	40.258
1974	40.92
1975	41.586
1976	42.087
1977	42.508
1978	42.931
1979	43.355
1980	43.78
1981	45.189
1982	46.997
1983	48.812
1984	50.634
1985	52.448
1986	53.93
1987	55.261
1988	56.587
1989	57.899
1990	59.203
1991	59.976
1992	60.518
1993	61.055
1994	61.59
1995	62.123
1996	62.653
1997	63.179
1998	63.703
1999	64.223
2000	64.741
2001	65.34
2002	65.974
2003	66.602
2004	67.225
2005	67.84
2006	68.45
2007	69.053
2008	69.651
2009	70.241
2010	70.825
2011	71.402
2012	71.974
2013	72.531
2014	73.077
2015	73.611
2016	74.134
2017	74.644