Year	Percent
1960	16.4
1961	17.475
1962	18.607
1963	19.794
1964	21.04
1965	22.338
1966	23.694
1967	25.106
1968	26.575
1969	28.093
1970	29.665
1971	31.287
1972	32.958
1973	34.669
1974	36.423
1975	38.213
1976	40.039
1977	41.888
1978	43.762
1979	45.654
1980	47.562
1981	49.471
1982	51.385
1983	53.294
1984	55.197
1985	57.079
1986	58.944
1987	60.783
1988	62.594
1989	64.366
1990	66.102
1991	67.796
1992	69.447
1993	71.044
1994	71.688
1995	71.669
1996	71.649
1997	71.629
1998	71.609
1999	71.589
2000	71.569
2001	71.549
2002	71.529
2003	71.509
2004	71.827
2005	72.4
2006	72.967
2007	73.527
2008	74.08
2009	74.624
2010	75.161
2011	76.183
2012	77.564
2013	78.882
2014	80.145
2015	81.35
2016	82.5
2017	83.56