Year	Percent
1960	44.547
1961	45.933
1962	47.327
1963	48.724
1964	50.126
1965	51.524
1966	52.921
1967	54.314
1968	55.702
1969	57.077
1970	58.328
1971	59.23
1972	60.127
1973	61.014
1974	61.896
1975	62.77
1976	63.637
1977	64.492
1978	65.34
1979	66.178
1980	67.835
1981	71.772
1982	75.406
1983	78.711
1984	81.684
1985	84.317
1986	86.637
1987	88.66
1988	90.413
1989	91.915
1990	92.942
1991	93.106
1992	93.267
1993	93.424
1994	93.578
1995	93.729
1996	93.876
1997	94.02
1998	94.161
1999	94.298
2000	94.387
2001	94.333
2002	94.278
2003	94.224
2004	94.168
2005	94.112
2006	94.056
2007	93.999
2008	93.942
2009	93.884
2010	93.825
2011	93.773
2012	93.726
2013	93.686
2014	93.651
2015	93.624
2016	93.602
2017	93.587