Year	Percent
1960	13.937
1961	14.488
1962	15.058
1963	15.647
1964	16.255
1965	16.88
1966	17.525
1967	18.19
1968	18.875
1969	19.577
1970	20.3
1971	21.584
1972	22.927
1973	24.324
1974	25.78
1975	27.292
1976	28.678
1977	29.469
1978	30.274
1979	31.091
1980	31.921
1981	32.761
1982	33.613
1983	34.476
1984	35.35
1985	36.232
1986	37.124
1987	37.941
1988	38.511
1989	39.082
1990	39.657
1991	40.235
1992	40.817
1993	41.399
1994	41.985
1995	42.573
1996	43.164
1997	43.755
1998	44.348
1999	44.944
2000	45.542
2001	46.139
2002	46.738
2003	47.338
2004	47.94
2005	48.541
2006	49.143
2007	49.746
2008	50.35
2009	50.954
2010	51.559
2011	52.164
2012	52.769
2013	53.373
2014	53.976
2015	54.578
2016	55.179
2017	55.777