Year	Percent
1960	27.323
1961	28.197
1962	29.089
1963	29.998
1964	30.924
1965	33.666
1966	36.698
1967	39.839
1968	43.07
1969	46.352
1970	49.671
1971	52.992
1972	56.292
1973	59.528
1974	61.237
1975	62.785
1976	64.31
1977	65.802
1978	67.265
1979	68.696
1980	70.094
1981	71.451
1982	72.773
1983	74.055
1984	75.3
1985	75.45
1986	75.505
1987	75.56
1988	75.614
1989	75.669
1990	75.723
1991	75.777
1992	75.832
1993	75.886
1994	75.94
1995	75.994
1996	76.048
1997	76.114
1998	76.193
1999	76.284
2000	76.387
2001	76.502
2002	76.629
2003	76.768
2004	76.919
2005	77.08
2006	77.253
2007	77.437
2008	77.632
2009	77.837
2010	78.052
2011	78.277
2012	78.512
2013	78.756
2014	79.009
2015	79.27
2016	79.54
2017	79.817