Year	Percent
1960	37.846
1961	37.912
1962	37.979
1963	38.046
1964	38.113
1965	38.18
1966	38.247
1967	38.314
1968	38.381
1969	38.448
1970	38.683
1971	39.47
1972	40.264
1973	41.061
1974	41.864
1975	42.671
1976	43.484
1977	44.297
1978	45.115
1979	45.935
1980	46.759
1981	47.793
1982	49.524
1983	51.257
1984	52.99
1985	54.71
1986	56.422
1987	58.118
1988	59.798
1989	61.45
1990	63.079
1991	64.482
1992	64.569
1993	64.656
1994	64.744
1995	64.831
1996	64.918
1997	65.005
1998	65.092
1999	65.179
2000	65.265
2001	65.382
2002	65.689
2003	65.994
2004	66.299
2005	66.601
2006	66.903
2007	67.203
2008	67.502
2009	67.799
2010	68.094
2011	68.389
2012	68.685
2013	68.982
2014	69.28
2015	69.579
2016	69.88
2017	70.181