Year	Percent
1960	31.25
1961	32.373
1962	33.518
1963	35.011
1964	36.872
1965	38.769
1966	40.702
1967	42.665
1968	44.654
1969	46.655
1970	48.669
1971	50.688
1972	52.707
1973	54.712
1974	56.704
1975	58.349
1976	59.897
1977	61.421
1978	62.925
1979	64.405
1980	65.86
1981	67.281
1982	68.673
1983	70.033
1984	71.36
1985	72.647
1986	73.9
1987	74.589
1988	75.267
1989	75.93
1990	76.583
1991	77.223
1992	77.851
1993	78.186
1994	78.429
1995	78.67
1996	78.91
1997	79.147
1998	79.383
1999	79.616
2000	79.848
2001	80.077
2002	80.304
2003	80.53
2004	80.754
2005	80.979
2006	81.204
2007	81.427
2008	81.649
2009	81.867
2010	82.084
2011	82.302
2012	82.52
2013	82.74
2014	82.96
2015	83.18
2016	83.401
2017	83.622