Year	Percent
1960	51.219
1961	52.741
1962	54.26
1963	55.771
1964	57.274
1965	58.759
1966	60.231
1967	61.688
1968	64.592
1969	67.384
1970	70.062
1971	72.609
1972	75.021
1973	77.28
1974	79.016
1975	80.554
1976	82.007
1977	83.37
1978	84.651
1979	85.85
1980	86.784
1981	87.117
1982	87.444
1983	87.763
1984	88.076
1985	88.381
1986	88.679
1987	88.971
1988	89.257
1989	89.535
1990	89.731
1991	89.697
1992	89.662
1993	89.627
1994	89.592
1995	89.557
1996	89.655
1997	89.784
1998	89.912
1999	90.038
2000	90.152
2001	90.235
2002	90.316
2003	90.398
2004	90.478
2005	90.558
2006	90.638
2007	90.716
2008	90.795
2009	90.872
2010	90.949
2011	91.028
2012	91.108
2013	91.19
2014	91.273
2015	91.357
2016	91.443
2017	91.53