Year	Percent
1960	16.29
1961	16.918
1962	17.566
1963	18.233
1964	18.92
1965	19.625
1966	20.351
1967	21.097
1968	21.863
1969	22.871
1970	24.088
1971	25.348
1972	26.654
1973	27.997
1974	28.96
1975	29.606
1976	30.262
1977	30.925
1978	31.596
1979	32.036
1980	32.284
1981	32.532
1982	32.782
1983	33.032
1984	33.284
1985	33.541
1986	33.828
1987	34.116
1988	34.405
1989	34.695
1990	34.987
1991	35.272
1992	35.555
1993	35.839
1994	36.123
1995	36.409
1996	37.386
1997	38.752
1998	40.138
1999	41.54
2000	42.958
2001	43.473
2002	43.492
2003	43.512
2004	43.532
2005	43.551
2006	44.03
2007	44.866
2008	45.706
2009	46.547
2010	47.39
2011	48.235
2012	49.082
2013	49.927
2014	50.773
2015	51.619
2016	52.45
2017	53.262