Year	Percent
1960	69.061
1961	69.668
1962	70.494
1963	71.307
1964	72.107
1965	72.892
1966	73.642
1967	74.155
1968	74.662
1969	75.161
1970	75.654
1971	76.09
1972	75.971
1973	75.851
1974	75.731
1975	75.611
1976	75.503
1977	75.543
1978	75.583
1979	75.623
1980	75.663
1981	75.712
1982	75.874
1983	76.035
1984	76.195
1985	76.354
1986	76.502
1987	76.522
1988	76.542
1989	76.562
1990	76.582
1991	76.62
1992	76.887
1993	77.152
1994	77.414
1995	77.675
1996	77.951
1997	78.34
1998	78.724
1999	79.103
2000	79.478
2001	79.81
2002	79.888
2003	79.967
2004	80.045
2005	80.122
2006	80.213
2007	80.396
2008	80.578
2009	80.758
2010	80.937
2011	81.096
2012	81.137
2013	81.178
2014	81.218
2015	81.259
2016	81.3
2017	81.35