Year	Percent
1960	11.52177
1961	11.82039
1962	12.07821
1963	12.33564
1964	12.59462
1965	12.8537
1966	13.11524
1967	13.37858
1968	13.64564
1969	13.91503
1970	14.18991
1971	14.46992
1972	14.75489
1973	14.98984
1974	15.20919
1975	15.4225
1976	15.63131
1977	15.83416
1978	16.03333
1979	16.22741
1980	16.41806
1981	16.54075
1982	16.53519
1983	16.52811
1984	16.52537
1985	16.52861
1986	16.5408
1987	16.56211
1988	16.5969
1989	16.64681
1990	16.71638
1991	16.80616
1992	16.91554
1993	17.03843
1994	17.17214
1995	17.31262
1996	17.45932
1997	17.6113
1998	17.74931
1999	17.85124
2000	17.96964
2001	18.10253
2002	18.24995
2003	18.40816
2004	18.57402
2005	18.74226
2006	18.91502
2007	19.09174
2008	19.27127
2009	19.44906
2010	19.62637
2011	19.80187
2012	19.97795
2013	20.15571
2014	20.34237
2015	20.54062
2016	20.75256
2017	20.97546