Year	Percent
1960	16.679
1961	16.948
1962	17.224
1963	17.503
1964	17.787
1965	18.073
1966	18.363
1967	18.657
1968	18.954
1969	19.255
1970	19.559
1971	19.9
1972	20.272
1973	20.649
1974	21.031
1975	21.419
1976	21.812
1977	22.209
1978	22.612
1979	23.02
1980	23.518
1981	24.995
1982	26.535
1983	28.133
1984	29.792
1985	31.5
1986	33.885
1987	36.355
1988	38.902
1989	41.504
1990	44.12
1991	45.044
1992	45.973
1993	46.902
1994	47.834
1995	48.768
1996	49.704
1997	50.637
1998	51.572
1999	52.505
2000	53.435
2001	54.291
2002	55.146
2003	55.997
2004	56.846
2005	57.689
2006	58.529
2007	59.363
2008	60.194
2009	61.016
2010	61.821
2011	62.322
2012	62.821
2013	63.317
2014	63.81
2015	64.3
2016	64.784
2017	65.261