Year	Percent
1960	49.92
1961	50.998
1962	52.653
1963	54.302
1964	55.944
1965	57.568
1966	59.179
1967	60.77
1968	62.34
1969	63.881
1970	65.396
1971	66.23
1972	66.725
1973	67.216
1974	67.703
1975	68.187
1976	68.667
1977	69.142
1978	69.614
1979	70.081
1980	70.545
1981	70.787
1982	70.918
1983	71.049
1984	71.18
1985	71.31
1986	71.44
1987	71.57
1988	71.699
1989	71.828
1990	71.956
1991	72.265
1992	72.665
1993	73.061
1994	73.453
1995	73.787
1996	74.062
1997	74.476
1998	75.027
1999	75.569
2000	76.02
2001	76.378
2002	76.733
2003	77.084
2004	77.399
2005	77.675
2006	77.95
2007	78.234
2008	78.526
2009	78.815
2010	79.102
2011	79.455
2012	79.874
2013	80.286
2014	80.692
2015	81.091
2016	81.485
2017	81.871