Year	Percent
1960	40.195
1961	41.162
1962	42.136
1963	43.116
1964	44.102
1965	45.091
1966	46.084
1967	47.081
1968	49.012
1969	51.609
1970	54.2
1971	54.702
1972	55.204
1973	55.703
1974	56.202
1975	56.7
1976	56.74
1977	56.78
1978	56.82
1979	56.86
1980	56.9
1981	57.049
1982	57.198
1983	57.346
1984	57.495
1985	57.644
1986	57.792
1987	57.94
1988	58.088
1989	58.236
1990	58.384
1991	58.531
1992	58.679
1993	58.826
1994	58.939
1995	59.018
1996	59.097
1997	59.176
1998	59.255
1999	59.334
2000	59.412
2001	59.491
2002	59.57
2003	59.648
2004	59.727
2005	59.805
2006	59.884
2007	59.962
2008	60.04
2009	60.195
2010	60.377
2011	60.557
2012	60.738
2013	60.918
2014	61.098
2015	61.277
2016	61.471
2017	61.678