Year	Percent
1960	47.892
1961	48.527
1962	48.926
1963	49.326
1964	49.727
1965	50.126
1966	50.526
1967	50.926
1968	51.326
1969	51.726
1970	52.125
1971	52.668
1972	53.324
1973	53.977
1974	54.629
1975	55.28
1976	55.929
1977	56.575
1978	57.22
1979	57.712
1980	58.086
1981	58.459
1982	58.831
1983	59.202
1984	59.573
1985	59.942
1986	60.31
1987	60.676
1988	61.043
1989	61.225
1990	61.27
1991	61.315
1992	61.359
1993	61.404
1994	61.449
1995	61.493
1996	61.538
1997	61.583
1998	61.627
1999	61.672
2000	61.716
2001	61.761
2002	61.787
2003	61.676
2004	61.564
2005	61.452
2006	61.341
2007	61.229
2008	61.116
2009	61.004
2010	60.892
2011	60.78
2012	60.654
2013	60.529
2014	60.403
2015	60.278
2016	60.178
2017	60.105