Year	Percent
1960	59.548
1961	60.063
1962	60.555
1963	61.044
1964	61.532
1965	62.016
1966	62.498
1967	62.978
1968	63.456
1969	63.93
1970	64.402
1971	65.295
1972	66.484
1973	67.649
1974	68.794
1975	69.917
1976	71.018
1977	72.092
1978	73.142
1979	74.167
1980	75.167
1981	75.481
1982	75.452
1983	75.423
1984	75.394
1985	75.365
1986	75.336
1987	75.307
1988	75.278
1989	75.249
1990	75.22
1991	75.158
1992	75.03
1993	74.901
1994	74.772
1995	74.643
1996	74.513
1997	74.382
1998	74.251
1999	74.12
2000	73.988
2001	73.877
2002	73.809
2003	73.74
2004	73.671
2005	73.602
2006	73.533
2007	73.463
2008	73.394
2009	73.324
2010	73.255
2011	73.185
2012	73.197
2013	73.29
2014	73.384
2015	73.477
2016	73.57
2017	73.675