Year	Percent
1960	42.899
1961	44.432
1962	45.978
1963	47.532
1964	49.093
1965	50.651
1966	51.868
1967	52.945
1968	54.02
1969	55.089
1970	56.154
1971	57.214
1972	58.269
1973	59.313
1974	60.351
1975	61.379
1976	62.398
1977	63.404
1978	64.178
1979	64.852
1980	65.521
1981	66.183
1982	66.839
1983	67.488
1984	68.132
1985	68.767
1986	69.396
1987	70.017
1988	70.072
1989	69.889
1990	69.706
1991	69.522
1992	69.337
1993	69.152
1994	68.966
1995	68.78
1996	68.593
1997	68.405
1998	68.388
1999	68.442
2000	68.496
2001	68.55
2002	68.604
2003	68.658
2004	68.712
2005	68.766
2006	68.819
2007	68.873
2008	68.927
2009	68.98
2010	69.103
2011	69.268
2012	69.432
2013	69.595
2014	69.758
2015	69.921
2016	70.094
2017	70.278