Year	Percent
1960	57.533
1961	58.304
1962	59.072
1963	59.836
1964	60.596
1965	61.348
1966	62.097
1967	62.839
1968	63.577
1969	64.307
1970	64.947
1971	65.483
1972	66.017
1973	66.545
1974	67.069
1975	67.589
1976	68.106
1977	68.617
1978	69.124
1979	69.497
1980	69.711
1981	69.924
1982	70.136
1983	70.347
1984	70.558
1985	70.767
1986	70.975
1987	71.183
1988	71.39
1989	71.415
1990	71.231
1991	71.046
1992	70.86
1993	70.674
1994	70.487
1995	70.3
1996	70.111
1997	69.923
1998	69.733
1999	69.543
2000	69.368
2001	69.242
2002	69.116
2003	68.989
2004	68.862
2005	68.735
2006	68.607
2007	68.479
2008	68.351
2009	68.223
2010	68.094
2011	67.965
2012	67.974
2013	68.122
2014	68.269
2015	68.416
2016	68.563
2017	68.717