Year	Percent
1960	8.748
1961	8.737
1962	8.725
1963	8.714
1964	8.702
1965	8.69
1966	8.679
1967	8.667
1968	8.656
1969	8.645
1970	8.633
1971	8.622
1972	8.61
1973	8.599
1974	8.587
1975	8.576
1976	8.565
1977	8.553
1978	8.542
1979	8.531
1980	8.519
1981	8.508
1982	8.497
1983	8.708
1984	9.261
1985	9.844
1986	10.461
1987	11.111
1988	11.797
1989	12.517
1990	13.276
1991	14.074
1992	14.912
1993	15.626
1994	15.749
1995	15.873
1996	15.997
1997	16.123
1998	16.249
1999	16.376
2000	16.504
2001	16.632
2002	16.761
2003	16.891
2004	17.023
2005	17.154
2006	17.287
2007	17.42
2008	17.555
2009	17.701
2010	17.86
2011	18.032
2012	18.217
2013	18.415
2014	18.626
2015	18.852
2016	19.092
2017	19.346