Year	Percent
1960	26.598
1961	27.241
1962	27.895
1963	28.558
1964	29.232
1965	29.913
1966	30.604
1967	31.304
1968	32.013
1969	32.729
1970	33.454
1971	34.271
1972	35.104
1973	35.944
1974	36.794
1975	37.652
1976	38.519
1977	39.391
1978	40.271
1979	41.158
1980	42.044
1981	42.807
1982	43.574
1983	44.345
1984	45.119
1985	45.893
1986	46.671
1987	47.45
1988	48.232
1989	49.012
1990	49.794
1991	50.576
1992	51.814
1993	53.109
1994	54.402
1995	55.688
1996	56.969
1997	58.237
1998	59.496
1999	60.742
2000	61.977
2001	62.922
2002	63.856
2003	64.78
2004	65.694
2005	66.594
2006	67.483
2007	68.36
2008	69.225
2009	70.075
2010	70.912
2011	71.609
2012	72.275
2013	72.93
2014	73.577
2015	74.213
2016	74.84
2017	75.447