Year	Percent
1960	10.404
1961	10.583
1962	10.765
1963	10.95
1964	11.138
1965	11.329
1966	11.522
1967	11.718
1968	11.918
1969	12.12
1970	12.325
1971	12.533
1972	12.744
1973	12.958
1974	13.175
1975	13.395
1976	13.619
1977	13.845
1978	14.074
1979	14.368
1980	14.74
1981	15.12
1982	15.508
1983	15.905
1984	16.31
1985	16.722
1986	17.143
1987	17.573
1988	18.011
1989	18.435
1990	18.715
1991	18.998
1992	19.285
1993	19.574
1994	19.868
1995	20.164
1996	20.464
1997	20.766
1998	21.073
1999	21.382
2000	21.673
2001	21.953
2002	22.235
2003	22.521
2004	22.809
2005	23.099
2006	23.392
2007	23.687
2008	23.985
2009	24.286
2010	24.462
2011	24.561
2012	24.661
2013	24.761
2014	24.861
2015	24.961
2016	25.062
2017	25.163