Year	Percent
1960	55.936
1961	56.453
1962	57.34
1963	58.222
1964	59.099
1965	59.969
1966	60.833
1967	61.691
1968	62.543
1969	63.385
1970	64.22
1971	64.951
1972	65.454
1973	65.952
1974	66.448
1975	66.94
1976	67.429
1977	67.912
1978	68.393
1979	68.87
1980	69.343
1981	69.746
1982	69.94
1983	70.134
1984	70.327
1985	70.518
1986	70.709
1987	70.9
1988	71.09
1989	71.279
1990	71.467
1991	71.635
1992	71.757
1993	71.878
1994	71.998
1995	72.119
1996	72.239
1997	72.358
1998	72.478
1999	72.597
2000	72.716
2001	72.913
2002	73.303
2003	73.689
2004	74.073
2005	74.452
2006	74.827
2007	75.199
2008	75.568
2009	75.932
2010	76.292
2011	76.649
2012	77.004
2013	77.355
2014	77.702
2015	78.046
2016	78.387
2017	78.724