Year	Percent
1960	58.45
1961	60.983
1962	63.462
1963	65.872
1964	68.205
1965	70.445
1966	72.593
1967	74.641
1968	76.588
1969	78.424
1970	80.155
1971	81.78
1972	83.301
1973	84.716
1974	86.032
1975	87.252
1976	88.382
1977	89.42
1978	90.378
1979	91.257
1980	92.064
1981	92.8
1982	93.474
1983	94.089
1984	94.649
1985	95.159
1986	95.325
1987	95.178
1988	95.027
1989	94.872
1990	94.712
1991	94.53
1992	94.324
1993	94.111
1994	93.89
1995	93.661
1996	93.425
1997	93.18
1998	92.927
1999	92.666
2000	92.395
2001	92.056
2002	91.641
2003	91.207
2004	90.751
2005	90.285
2006	89.807
2007	89.308
2008	89.004
2009	88.912
2010	88.819
2011	88.726
2012	88.631
2013	88.537
2014	88.441
2015	88.345
2016	88.248
2017	88.15