Year	Percent
1960	56.823
1961	57.114
1962	57.405
1963	57.696
1964	57.987
1965	58.276
1966	58.565
1967	58.853
1968	59.142
1969	59.429
1970	59.715
1971	60.001
1972	60.287
1973	60.571
1974	60.855
1975	61.138
1976	61.421
1977	61.702
1978	61.983
1979	62.263
1980	62.542
1981	63.409
1982	64.422
1983	65.421
1984	66.409
1985	67.38
1986	68.188
1987	68.541
1988	68.892
1989	69.24
1990	69.593
1991	69.966
1992	70.337
1993	70.704
1994	71.069
1995	71.431
1996	71.306
1997	71.066
1998	70.825
1999	70.582
2000	70.338
2001	70.093
2002	69.847
2003	69.6
2004	70.383
2005	71.152
2006	71.91
2007	72.655
2008	73.389
2009	74.109
2010	74.817
2011	75.511
2012	76.194
2013	76.862
2014	77.517
2015	78.159
2016	78.775
2017	79.365