Year	Percent
1960	15.593
1961	15.975
1962	16.365
1963	16.762
1964	17.168
1965	17.58
1966	18.001
1967	18.429
1968	18.866
1969	19.31
1970	19.762
1971	20.222
1972	20.323
1973	20.35
1974	20.377
1975	20.404
1976	20.431
1977	20.459
1978	20.486
1979	20.513
1980	20.541
1981	20.568
1982	20.595
1983	21.36
1984	22.297
1985	23.261
1986	24.255
1987	25.277
1988	26.329
1989	27.405
1990	28.51
1991	29.641
1992	30.8
1993	31.39
1994	31.987
1995	32.59
1996	33.2
1997	33.792
1998	34.389
1999	34.992
2000	35.6
2001	37.251
2002	38.935
2003	40.645
2004	41.679
2005	42.638
2006	43.604
2007	44.574
2008	45.551
2009	46.528
2010	47.509
2011	48.492
2012	49.477
2013	50.46
2014	51.444
2015	52.427
2016	53.396
2017	54.346