Year	Percent
1960	14.7
1961	15.031
1962	15.369
1963	15.714
1964	16.064
1965	16.421
1966	16.784
1967	17.153
1968	17.529
1969	17.911
1970	18.3
1971	18.396
1972	18.492
1973	18.588
1974	18.685
1975	18.782
1976	18.88
1977	18.978
1978	19.076
1979	19.175
1980	19.247
1981	19.309
1982	19.372
1983	19.435
1984	19.498
1985	19.561
1986	19.625
1987	19.688
1988	19.752
1989	19.891
1990	20.257
1991	20.629
1992	21.006
1993	21.387
1994	21.774
1995	22.166
1996	22.563
1997	22.965
1998	23.371
1999	23.817
2000	24.374
2001	24.937
2002	25.511
2003	26.092
2004	26.683
2005	27.281
2006	27.888
2007	28.504
2008	29.128
2009	29.762
2010	30.417
2011	31.08
2012	31.752
2013	32.429
2014	33.115
2015	33.809
2016	34.51
2017	35.213