Year	Percent
1960	5.768
1961	6.038
1962	6.321
1963	6.616
1964	6.924
1965	7.245
1966	7.579
1967	7.928
1968	8.292
1969	8.67
1970	8.917
1971	8.958
1972	8.999
1973	9.041
1974	9.083
1975	9.125
1976	9.271
1977	9.583
1978	9.904
1979	10.235
1980	10.576
1981	10.927
1982	11.288
1983	11.659
1984	12.041
1985	12.433
1986	12.837
1987	13.111
1988	13.298
1989	13.486
1990	13.677
1991	13.87
1992	14.065
1993	14.262
1994	14.462
1995	14.664
1996	14.869
1997	15.076
1998	15.285
1999	15.496
2000	15.813
2001	16.201
2002	16.597
2003	17.001
2004	17.413
2005	17.832
2006	18.259
2007	18.694
2008	19.138
2009	19.589
2010	20.048
2011	20.508
2012	20.97
2013	21.433
2014	21.896
2015	22.36
2016	22.823
2017	23.286