Year	Percent
1960	73.611
1961	74.217
1962	74.767
1963	75.309
1964	75.844
1965	76.369
1966	76.888
1967	77.398
1968	77.901
1969	78.394
1970	78.88
1971	79.321
1972	79.743
1973	80.158
1974	80.567
1975	80.969
1976	81.366
1977	81.755
1978	82.139
1979	82.516
1980	82.887
1981	83.313
1982	83.759
1983	84.194
1984	84.621
1985	85.038
1986	85.445
1987	85.843
1988	86.233
1989	86.613
1990	86.984
1991	87.328
1992	87.542
1993	87.752
1994	87.96
1995	88.164
1996	88.366
1997	88.564
1998	88.759
1999	88.952
2000	89.142
2001	89.329
2002	89.51
2003	89.686
2004	89.86
2005	90.031
2006	90.2
2007	90.366
2008	90.53
2009	90.691
2010	90.849
2011	90.99
2012	91.121
2013	91.249
2014	91.377
2015	91.503
2016	91.627
2017	91.749