Year	Percent
1960	36.807
1961	37.399
1962	38.044
1963	38.694
1964	39.349
1965	40.006
1966	40.667
1967	41.332
1968	42.001
1969	42.671
1970	43.345
1971	43.751
1972	44.078
1973	44.405
1974	44.733
1975	45.061
1976	45.39
1977	45.718
1978	46.048
1979	46.377
1980	46.708
1981	47.038
1982	47.268
1983	47.476
1984	47.684
1985	47.891
1986	48.099
1987	48.307
1988	48.515
1989	48.723
1990	48.931
1991	49.139
1992	49.347
1993	49.555
1994	49.764
1995	50.104
1996	50.473
1997	50.841
1998	51.21
1999	51.578
2000	51.947
2001	52.314
2002	52.682
2003	53.049
2004	53.417
2005	53.782
2006	54.146
2007	54.511
2008	54.875
2009	55.237
2010	55.6
2011	54.58
2012	53.555
2013	52.529
2014	51.5
2015	52.168
2016	52.835
2017	53.5