Year	Percent
1960	10.285
1961	10.293
1962	10.333
1963	10.497
1964	10.662
1965	10.83
1966	11
1967	12.098
1968	13.291
1969	14.579
1970	15.97
1971	18.717
1972	21.818
1973	25.263
1974	29.055
1975	4.477
1976	4.82
1977	5.186
1978	5.578
1979	5.999
1980	9.898
1981	12.742
1982	13.031
1983	13.325
1984	13.626
1985	13.931
1986	14.242
1987	14.559
1988	14.883
1989	15.211
1990	15.546
1991	15.887
1992	16.234
1993	16.587
1994	16.946
1995	17.311
1996	17.683
1997	18.06
1998	18.355
1999	18.47
2000	18.586
2001	18.703
2002	18.82
2003	18.937
2004	19.055
2005	19.174
2006	19.293
2007	19.413
2008	19.571
2009	19.93
2010	20.294
2011	20.663
2012	21.037
2013	21.415
2014	21.799
2015	22.188
2016	22.582
2017	22.98