Year	Percent
1960	12.551
1961	13.348
1962	14.189
1963	15.074
1964	16.004
1965	16.978
1966	18
1967	18.34
1968	18.685
1969	19.035
1970	19.39
1971	19.75
1972	20.115
1973	20.484
1974	20.859
1975	21.239
1976	21.625
1977	22.014
1978	22.41
1979	22.81
1980	23.215
1981	23.649
1982	24.095
1983	24.548
1984	25.006
1985	25.469
1986	25.938
1987	26.413
1988	26.893
1989	27.378
1990	27.869
1991	28.366
1992	28.435
1993	28.39
1994	28.346
1995	28.301
1996	28.257
1997	28.213
1998	28.168
1999	28.124
2000	28.08
2001	28.036
2002	27.992
2003	27.947
2004	27.903
2005	27.875
2006	27.863
2007	27.867
2008	27.886
2009	27.921
2010	27.973
2011	28.04
2012	28.123
2013	28.223
2014	28.338
2015	28.47
2016	28.619
2017	28.784