Year	Percent
1960	90.129
1961	90.069
1962	90.008
1963	89.948
1964	89.886
1965	89.825
1966	89.764
1967	89.702
1968	89.682
1969	89.69
1970	89.699
1971	89.708
1972	89.716
1973	89.725
1974	89.734
1975	89.742
1976	89.751
1977	89.76
1978	89.768
1979	89.777
1980	89.785
1981	89.794
1982	89.803
1983	89.811
1984	89.82
1985	89.828
1986	89.907
1987	90.028
1988	90.147
1989	90.265
1990	90.381
1991	90.497
1992	90.611
1993	90.723
1994	90.835
1995	90.946
1996	91.181
1997	91.492
1998	91.793
1999	92.085
2000	92.368
2001	92.641
2002	92.905
2003	93.16
2004	93.407
2005	93.645
2006	93.784
2007	93.857
2008	93.929
2009	94.001
2010	94.072
2011	94.141
2012	94.211
2013	94.279
2014	94.347
2015	94.414
2016	94.48
2017	94.546