Year	Percent
1960	71.384
1961	71.697
1962	71.762
1963	71.826
1964	71.891
1965	71.955
1966	72.019
1967	72.083
1968	72.147
1969	72.211
1970	72.274
1971	72.332
1972	72.389
1973	72.446
1974	72.503
1975	72.56
1976	72.617
1977	72.674
1978	72.73
1979	72.787
1980	72.844
1981	72.989
1982	73.107
1983	73.103
1984	72.943
1985	72.711
1986	72.617
1987	72.836
1988	72.999
1989	72.976
1990	73.118
1991	73.269
1992	73.36
1993	73.497
1994	73.71
1995	73.922
1996	74.133
1997	74.342
1998	74.551
1999	74.758
2000	74.965
2001	75.17
2002	75.374
2003	75.577
2004	75.779
2005	75.98
2006	76.179
2007	76.378
2008	76.575
2009	76.771
2010	76.966
2011	77.16
2012	77.17
2013	77.18
2014	77.19
2015	77.2
2016	77.224
2017	77.261