Year	Percent
1960	42.281
1961	43.217
1962	44.16
1963	45.389
1964	46.783
1965	48.178
1966	49.578
1967	50.979
1968	52.38
1969	53.773
1970	55.162
1971	56.203
1972	56.718
1973	57.231
1974	57.742
1975	58.252
1976	58.76
1977	59.171
1978	59.115
1979	59.059
1980	59.003
1981	58.948
1982	58.892
1983	58.836
1984	58.743
1985	58.635
1986	58.527
1987	58.419
1988	58.31
1989	58.094
1990	57.852
1991	57.61
1992	57.367
1993	57.124
1994	56.881
1995	56.637
1996	56.393
1997	56.285
1998	56.205
1999	56.126
2000	56.047
2001	55.967
2002	55.888
2003	56.242
2004	56.831
2005	57.417
2006	58.002
2007	58.585
2008	59.166
2009	59.742
2010	60.317
2011	60.889
2012	61.459
2013	61.583
2014	61.633
2015	61.683
2016	61.734
2017	61.784