Year	Percent
1960	50.753
1961	51.59
1962	52.428
1963	53.265
1964	54.1
1965	54.931
1966	55.761
1967	56.587
1968	57.411
1969	58.229
1970	59.021
1971	59.779
1972	60.532
1973	61.279
1974	62.022
1975	62.759
1976	63.491
1977	64.215
1978	64.934
1979	65.646
1980	66.339
1981	66.87
1982	67.396
1983	67.919
1984	68.437
1985	68.95
1986	69.459
1987	69.963
1988	70.462
1989	70.956
1990	71.419
1991	71.816
1992	72.209
1993	72.598
1994	72.985
1995	73.368
1996	73.67
1997	73.929
1998	74.186
1999	74.441
2000	74.722
2001	75.045
2002	75.365
2003	75.682
2004	75.997
2005	76.308
2006	76.616
2007	76.92
2008	77.222
2009	77.52
2010	77.815
2011	78.111
2012	78.405
2013	78.699
2014	78.993
2015	79.285
2016	79.577
2017	79.867