Year	Percent
1960	14.586
1961	14.821
1962	15.058
1963	15.299
1964	15.543
1965	15.789
1966	16.039
1967	16.293
1968	16.549
1969	16.809
1970	17.071
1971	17.338
1972	17.779
1973	18.28
1974	18.793
1975	19.317
1976	19.853
1977	20.398
1978	20.956
1979	21.524
1980	22.104
1981	22.815
1982	23.606
1983	24.415
1984	25.244
1985	26.089
1986	26.953
1987	27.835
1988	28.736
1989	29.651
1990	30.584
1991	31.613
1992	32.703
1993	33.808
1994	34.933
1995	36.076
1996	37.235
1997	38.406
1998	39.593
1999	40.792
2000	42.002
2001	42.783
2002	43.568
2003	44.356
2004	45.149
2005	45.942
2006	46.738
2007	47.535
2008	48.335
2009	49.134
2010	49.914
2011	50.595
2012	51.276
2013	51.955
2014	52.635
2015	53.313
2016	53.989
2017	54.659