Year	Percent
1960	43.401
1961	45.228
1962	47.07
1963	48.92
1964	50.776
1965	52.625
1966	54.469
1967	56.3
1968	58.118
1969	59.908
1970	61.675
1971	63.412
1972	63.736
1973	63.889
1974	64.041
1975	64.193
1976	64.345
1977	64.496
1978	64.647
1979	64.798
1980	64.949
1981	65.099
1982	65.249
1983	65.399
1984	65.548
1985	65.697
1986	65.846
1987	65.994
1988	66.143
1989	66.29
1990	66.438
1991	66.585
1992	67.078
1993	67.604
1994	68.126
1995	68.644
1996	69.158
1997	69.666
1998	70.17
1999	70.669
2000	71.164
2001	71.652
2002	72.046
2003	72.421
2004	72.794
2005	73.163
2006	73.529
2007	73.892
2008	74.252
2009	74.608
2010	74.961
2011	75.31
2012	75.656
2013	75.997
2014	76.332
2015	76.663
2016	76.99
2017	77.312