Year	Percent
1960	39.46
1961	40.443
1962	41.435
1963	42.434
1964	43.441
1965	44.45
1966	45.465
1967	46.484
1968	47.507
1969	48.53
1970	49.555
1971	50.719
1972	51.968
1973	53.21
1974	54.451
1975	55.685
1976	56.915
1977	58.132
1978	59.342
1979	60.361
1980	61.158
1981	61.946
1982	62.73
1983	63.506
1984	64.277
1985	65.038
1986	65.793
1987	66.54
1988	67.28
1989	67.642
1990	67.583
1991	67.523
1992	67.464
1993	67.404
1994	67.345
1995	67.285
1996	67.225
1997	67.166
1998	67.106
1999	67.046
2000	66.986
2001	66.919
2002	66.828
2003	66.737
2004	66.646
2005	66.635
2006	66.706
2007	66.777
2008	66.848
2009	66.842
2010	66.757
2011	66.741
2012	66.864
2013	66.986
2014	67.108
2015	67.23
2016	67.366
2017	67.516