Year	Percent
1960	3.596
1961	3.792
1962	3.999
1963	4.217
1964	4.446
1965	4.687
1966	4.94
1967	5.207
1968	5.487
1969	5.781
1970	6.089
1971	6.413
1972	6.754
1973	7.11
1974	7.484
1975	7.876
1976	8.288
1977	8.717
1978	9.168
1979	9.639
1980	10.132
1981	10.646
1982	11.184
1983	11.745
1984	12.332
1985	12.941
1986	13.577
1987	14.24
1988	14.929
1989	15.645
1990	16.388
1991	17.161
1992	17.962
1993	18.791
1994	19.649
1995	20.537
1996	21.456
1997	22.401
1998	23.377
1999	24.383
2000	25.418
2001	26.48
2002	27.57
2003	28.688
2004	29.835
2005	30.965
2006	31.712
2007	32.469
2008	33.236
2009	34.01
2010	34.793
2011	35.585
2012	36.371
2013	37.149
2014	37.918
2015	38.678
2016	39.428
2017	40.167