Year	Percent
1960	5.793
1961	5.896
1962	6
1963	6.259
1964	6.529
1965	6.809
1966	7.1
1967	7.493
1968	7.906
1969	8.339
1970	8.794
1971	9.271
1972	9.772
1973	10.296
1974	10.845
1975	11.419
1976	12.021
1977	12.648
1978	13.025
1979	13.233
1980	13.443
1981	13.656
1982	13.871
1983	14.09
1984	14.312
1985	14.536
1986	14.763
1987	14.993
1988	15.209
1989	15.289
1990	15.368
1991	15.449
1992	15.529
1993	15.61
1994	15.691
1995	15.773
1996	15.855
1997	15.937
1998	16.02
1999	16.103
2000	16.186
2001	16.26
2002	16.255
2003	16.251
2004	16.247
2005	16.242
2006	16.238
2007	16.234
2008	16.229
2009	16.225
2010	16.221
2011	16.216
2012	16.212
2013	16.208
2014	16.219
2015	16.247
2016	16.29
2017	16.35