Year	Percent
1960	19.04
1961	19.686
1962	20.437
1963	21.209
1964	22.004
1965	22.817
1966	23.652
1967	24.509
1968	25.387
1969	26.283
1970	27.201
1971	28.094
1972	28.875
1973	29.666
1974	30.471
1975	31.288
1976	32.118
1977	32.957
1978	33.808
1979	34.67
1980	35.543
1981	36.282
1982	36.607
1983	36.933
1984	37.261
1985	37.589
1986	37.919
1987	38.25
1988	38.583
1989	38.915
1990	39.249
1991	39.578
1992	39.887
1993	40.196
1994	40.506
1995	40.817
1996	41.13
1997	41.442
1998	41.755
1999	42.069
2000	42.384
2001	42.698
2002	43.014
2003	43.33
2004	43.648
2005	43.965
2006	44.282
2007	44.601
2008	44.92
2009	45.239
2010	45.558
2011	45.878
2012	46.199
2013	46.519
2014	46.84
2015	47.173
2016	47.518
2017	47.876