Year	Percent
1960	61.88
1961	62.607
1962	63.489
1963	64.702
1964	65.898
1965	67.071
1966	68.225
1967	69.357
1968	70.238
1969	70.648
1970	71.055
1971	71.459
1972	71.86
1973	72.256
1974	72.65
1975	72.926
1976	72.997
1977	73.068
1978	73.14
1979	73.211
1980	73.282
1981	73.352
1982	73.425
1983	73.5
1984	73.575
1985	73.65
1986	73.725
1987	73.8
1988	73.875
1989	73.95
1990	74.056
1991	74.229
1992	74.401
1993	74.572
1994	74.743
1995	74.912
1996	75.082
1997	75.25
1998	75.417
1999	75.614
2000	75.871
2001	76.127
2002	76.38
2003	76.632
2004	76.883
2005	77.13
2006	77.377
2007	77.621
2008	77.868
2009	78.117
2010	78.369
2011	78.622
2012	78.878
2013	79.135
2014	79.394
2015	79.655
2016	79.917
2017	80.18