Year	Percent
1960	58.401
1961	58.588
1962	58.776
1963	58.963
1964	59.15
1965	59.337
1966	59.523
1967	59.709
1968	59.895
1969	60.081
1970	60.266
1971	60.976
1972	61.8
1973	62.614
1974	63.423
1975	64.224
1976	65.018
1977	65.802
1978	66.579
1979	67.346
1980	68.106
1981	68.853
1982	69.414
1983	69.925
1984	70.433
1985	70.933
1986	71.43
1987	71.921
1988	72.408
1989	72.888
1990	73.364
1991	73.676
1992	73.827
1993	73.977
1994	74.127
1995	74.277
1996	74.426
1997	74.618
1998	74.855
1999	75.089
2000	75.323
2001	75.555
2002	75.785
2003	75.947
2004	76.041
2005	76.134
2006	76.227
2007	76.32
2008	76.413
2009	76.505
2010	76.597
2011	76.689
2012	76.781
2013	76.827
2014	76.861
2015	76.896
2016	76.93
2017	76.977