Year	Percent
1960	4.39
1961	4.489
1962	4.59
1963	4.694
1964	4.8
1965	4.908
1966	5.018
1967	5.249
1968	5.505
1969	5.773
1970	6.053
1971	6.345
1972	6.651
1973	6.97
1974	7.304
1975	7.652
1976	8.015
1977	8.394
1978	8.636
1979	8.841
1980	9.05
1981	9.263
1982	9.481
1983	9.704
1984	9.931
1985	10.163
1986	10.4
1987	10.641
1988	10.934
1989	11.243
1990	11.56
1991	11.885
1992	12.218
1993	12.558
1994	12.907
1995	13.263
1996	13.629
1997	14.002
1998	14.384
1999	14.522
2000	14.61
2001	14.698
2002	14.786
2003	14.875
2004	14.964
2005	15.054
2006	15.144
2007	15.235
2008	15.326
2009	15.429
2010	15.544
2011	15.672
2012	15.812
2013	15.966
2014	16.132
2015	16.313
2016	16.506
2017	16.714