Year	Percent
1960	6.433
1961	6.65
1962	6.874
1963	7.104
1964	7.342
1965	7.587
1966	7.84
1967	8.1
1968	8.26
1969	8.422
1970	8.588
1971	8.756
1972	8.928
1973	9.102
1974	9.279
1975	9.46
1976	9.644
1977	9.83
1978	10.02
1979	10.214
1980	10.41
1981	10.61
1982	10.813
1983	11.02
1984	11.232
1985	11.453
1986	11.679
1987	11.909
1988	12.143
1989	12.38
1990	12.621
1991	12.867
1992	13.116
1993	13.37
1994	13.627
1995	13.827
1996	14.006
1997	14.186
1998	14.369
1999	14.553
2000	14.74
2001	14.927
2002	15.118
2003	15.31
2004	15.504
2005	15.7
2006	15.899
2007	16.116
2008	16.51
2009	16.91
2010	17.319
2011	17.735
2012	18.16
2013	18.575
2014	18.998
2015	19.428
2016	19.866
2017	20.31