Year	Percent
1960	3.512
1961	3.96
1962	4.464
1963	5.029
1964	5.661
1965	6.366
1966	7.065
1967	7.425
1968	7.802
1969	8.196
1970	8.608
1971	9.039
1972	9.474
1973	9.905
1974	10.355
1975	10.822
1976	11.213
1977	11.272
1978	11.331
1979	11.39
1980	11.45
1981	11.51
1982	11.57
1983	11.63
1984	11.691
1985	11.752
1986	11.905
1987	12.394
1988	12.901
1989	13.425
1990	13.967
1991	14.527
1992	15.106
1993	15.703
1994	16.319
1995	16.955
1996	17.571
1997	18.049
1998	18.538
1999	19.038
2000	19.548
2001	20.067
2002	20.597
2003	21.137
2004	21.689
2005	22.249
2006	22.82
2007	23.304
2008	23.796
2009	24.293
2010	24.798
2011	25.31
2012	25.704
2013	26.101
2014	26.502
2015	26.908
2016	27.317
2017	27.73