Year	Percent
1960	34.209
1961	34.894
1962	35.586
1963	36.284
1964	36.989
1965	37.697
1966	38.345
1967	38.835
1968	39.328
1969	39.822
1970	40.319
1971	40.818
1972	41.319
1973	41.821
1974	42.325
1975	42.831
1976	43.339
1977	43.942
1978	44.649
1979	45.358
1980	46.07
1981	46.781
1982	47.495
1983	48.21
1984	48.926
1985	49.641
1986	50.357
1987	51.073
1988	51.79
1989	52.504
1990	53.217
1991	53.93
1992	54.227
1993	54.074
1994	53.922
1995	53.769
1996	53.616
1997	53.463
1998	53.311
1999	53.158
2000	53.004
2001	52.852
2002	52.78
2003	52.912
2004	53.043
2005	53.174
2006	53.305
2007	53.436
2008	53.567
2009	53.698
2010	53.829
2011	53.96
2012	53.974
2013	53.937
2014	53.9
2015	53.887
2016	53.9
2017	53.936