Year	Percent
1960	31.601
1961	32.318
1962	33.045
1963	33.78
1964	34.524
1965	35.273
1966	36.031
1967	36.796
1968	37.569
1969	38.345
1970	39.129
1971	39.919
1972	40.715
1973	41.514
1974	42.357
1975	43.264
1976	44.177
1977	45.091
1978	46.01
1979	46.932
1980	47.857
1981	48.781
1982	49.707
1983	50.633
1984	51.56
1985	52.221
1986	52.642
1987	53.064
1988	53.485
1989	53.905
1990	54.324
1991	54.743
1992	55.162
1993	55.58
1994	55.996
1995	56.413
1996	56.832
1997	57.299
1998	57.766
1999	58.231
2000	58.695
2001	59.156
2002	59.617
2003	60.076
2004	60.533
2005	60.988
2006	61.441
2007	61.893
2008	62.346
2009	62.8
2010	63.256
2011	63.712
2012	64.169
2013	64.627
2014	65.085
2015	65.543
2016	66.001
2017	66.459