Year	Percent
1960	15.41
1961	15.633
1962	15.858
1963	16.087
1964	16.318
1965	16.551
1966	16.788
1967	17.027
1968	17.269
1969	17.513
1970	17.76
1971	18.151
1972	18.549
1973	18.952
1974	19.363
1975	19.78
1976	20.205
1977	20.636
1978	21.074
1979	21.518
1980	21.97
1981	22.671
1982	23.389
1983	24.122
1984	24.872
1985	25.635
1986	26.414
1987	27.209
1988	28.019
1989	28.842
1990	29.68
1991	30.176
1992	30.677
1993	31.182
1994	31.691
1995	32.205
1996	32.725
1997	33.247
1998	33.773
1999	34.304
2000	34.84
2001	35.669
2002	36.508
2003	37.356
2004	38.212
2005	39.074
2006	39.943
2007	40.819
2008	41.702
2009	42.588
2010	43.48
2011	44.366
2012	45.246
2013	46.118
2014	46.982
2015	47.838
2016	48.683
2017	49.519