Year	Percent
1960	18.788
1961	19.502
1962	20.237
1963	20.993
1964	21.77
1965	22.566
1966	23.383
1967	24.22
1968	25.079
1969	25.955
1970	26.853
1971	27.769
1972	28.706
1973	29.659
1974	30.632
1975	31.622
1976	32.63
1977	33.652
1978	34.69
1979	35.744
1980	36.813
1981	37.892
1982	38.984
1983	40.088
1984	41.204
1985	42.325
1986	43.456
1987	44.594
1988	45.739
1989	46.886
1990	48.037
1991	49.165
1992	50.218
1993	51.267
1994	52.317
1995	53.365
1996	54.412
1997	55.451
1998	56.488
1999	57.518
2000	58.544
2001	59.559
2002	60.568
2003	61.568
2004	62.127
2005	62.465
2006	62.803
2007	63.139
2008	63.475
2009	63.808
2010	64.14
2011	64.472
2012	64.804
2013	65.137
2014	65.471
2015	65.806
2016	66.141
2017	66.477