Year	Percent
1960	53.731
1961	54.626
1962	55.518
1963	56.408
1964	57.294
1965	58.173
1966	59.048
1967	59.918
1968	60.783
1969	61.638
1970	62.471
1971	63.278
1972	64.078
1973	64.869
1974	65.652
1975	66.427
1976	67.194
1977	67.95
1978	68.698
1979	69.305
1980	69.751
1981	70.193
1982	70.631
1983	71.066
1984	71.497
1985	71.923
1986	72.346
1987	72.765
1988	73.181
1989	73.398
1990	73.394
1991	73.389
1992	73.385
1993	73.381
1994	73.376
1995	73.372
1996	73.367
1997	73.363
1998	73.359
1999	73.354
2000	73.35
2001	73.346
2002	73.341
2003	73.373
2004	73.418
2005	73.463
2006	73.508
2007	73.553
2008	73.598
2009	73.642
2010	73.687
2011	73.732
2012	73.791
2013	73.863
2014	73.95
2015	74.05
2016	74.164
2017	74.292