Year	Percent
1960	50.147
1961	51.23
1962	52.314
1963	53.396
1964	54.475
1965	55.548
1966	56.617
1967	57.68
1968	58.738
1969	59.784
1970	61.92
1971	67.153
1972	71.999
1973	76.371
1974	80.252
1975	83.632
1976	86.534
1977	88.983
1978	91.036
1979	92.737
1980	93.753
1981	93.491
1982	93.219
1983	92.937
1984	92.643
1985	92.339
1986	92.023
1987	91.695
1988	91.354
1989	91.002
1990	90.796
1991	91.061
1992	91.32
1993	91.571
1994	91.816
1995	92.055
1996	92.287
1997	92.513
1998	92.733
1999	92.946
2000	93.129
2001	93.233
2002	93.334
2003	93.435
2004	93.534
2005	93.631
2006	93.727
2007	93.822
2008	93.916
2009	94.008
2010	94.099
2011	94.188
2012	94.277
2013	94.364
2014	94.449
2015	94.534
2016	94.617
2017	94.699