Year	Percent
1960	10.098
1961	10.94
1962	11.844
1963	12.811
1964	13.847
1965	14.95
1966	16.125
1967	17.374
1968	18.7
1969	20.098
1970	21.28
1971	21.602
1972	21.929
1973	22.258
1974	22.591
1975	22.927
1976	23.268
1977	23.611
1978	23.958
1979	24.308
1980	24.663
1981	25.02
1982	25.391
1983	25.778
1984	26.169
1985	26.562
1986	26.96
1987	27.362
1988	27.768
1989	28.176
1990	28.589
1991	29.005
1992	29.425
1993	29.848
1994	30.275
1995	30.705
1996	31.139
1997	31.576
1998	32.016
1999	32.46
2000	32.907
2001	33.356
2002	33.809
2003	34.265
2004	34.724
2005	35.185
2006	35.649
2007	36.116
2008	36.587
2009	37.058
2010	37.533
2011	38.032
2012	38.546
2013	39.061
2014	39.579
2015	40.1
2016	40.628
2017	41.162