Year	Percent
1960	5.135
1961	5.278
1962	5.498
1963	5.727
1964	5.964
1965	6.211
1966	6.467
1967	6.733
1968	7.009
1969	7.296
1970	7.593
1971	7.901
1972	8.221
1973	8.553
1974	9.034
1975	9.836
1976	10.701
1977	11.63
1978	12.629
1979	13.701
1980	14.851
1981	15.801
1982	16.212
1983	16.631
1984	17.06
1985	17.496
1986	17.941
1987	18.395
1988	18.859
1989	19.33
1990	19.811
1991	20.257
1992	20.61
1993	20.966
1994	21.328
1995	21.693
1996	22.064
1997	22.438
1998	22.818
1999	23.202
2000	23.59
2001	24.096
2002	24.756
2003	25.429
2004	26.114
2005	26.809
2006	27.517
2007	28.237
2008	28.968
2009	29.709
2010	30.462
2011	31.225
2012	31.993
2013	32.762
2014	33.535
2015	34.308
2016	35.083
2017	35.858