Year	Percent
1960	17.679
1961	18.911
1962	20.21
1963	21.575
1964	23.007
1965	24.5
1966	25.206
1967	25.926
1968	26.659
1969	27.404
1970	28.163
1971	28.934
1972	29.719
1973	30.513
1974	31.321
1975	32.229
1976	33.594
1977	34.983
1978	36.4
1979	36.614
1980	36.829
1981	37.044
1982	37.26
1983	37.476
1984	37.693
1985	37.91
1986	38.127
1987	38.346
1988	38.608
1989	38.976
1990	39.345
1991	39.716
1992	40.088
1993	40.461
1994	40.835
1995	41.21
1996	41.587
1997	41.964
1998	42.342
1999	42.741
2000	43.155
2001	43.568
2002	43.983
2003	44.399
2004	44.817
2005	45.234
2006	45.652
2007	46.071
2008	46.491
2009	46.91
2010	47.33
2011	47.751
2012	48.172
2013	48.593
2014	49.014
2015	49.444
2016	49.881
2017	50.326