Year	Percent
1960	50.325
1961	51.448
1962	52.619
1963	53.787
1964	54.952
1965	56.109
1966	57.26
1967	58.404
1968	59.54
1969	60.664
1970	61.777
1971	62.878
1972	63.968
1973	65.04
1974	66.099
1975	67.142
1976	68.17
1977	69.178
1978	70.169
1979	71.141
1980	72.096
1981	73.028
1982	73.942
1983	74.504
1984	74.715
1985	74.925
1986	75.134
1987	75.341
1988	75.548
1989	75.753
1990	75.957
1991	76.114
1992	76.161
1993	76.207
1994	76.254
1995	76.3
1996	76.347
1997	76.393
1998	76.439
1999	76.485
2000	76.532
2001	76.578
2002	76.624
2003	76.67
2004	76.716
2005	76.761
2006	76.807
2007	76.853
2008	76.899
2009	76.944
2010	76.999
2011	77.064
2012	77.138
2013	77.222
2014	77.314
2015	77.417
2016	77.528
2017	77.648