Year	Percent
1960	10.746
1961	11.232
1962	11.737
1963	12.262
1964	12.808
1965	13.373
1966	13.959
1967	14.567
1968	15.197
1969	15.848
1970	16.523
1971	17.22
1972	17.941
1973	18.548
1974	18.744
1975	18.943
1976	19.143
1977	19.344
1978	19.547
1979	19.752
1980	19.959
1981	20.166
1982	20.376
1983	20.901
1984	21.903
1985	22.937
1986	24.005
1987	25.107
1988	26.244
1989	27.41
1990	28.61
1991	29.84
1992	31.103
1993	32.127
1994	32.179
1995	32.232
1996	32.284
1997	32.337
1998	32.39
1999	32.442
2000	32.495
2001	32.548
2002	32.601
2003	32.654
2004	32.707
2005	32.76
2006	32.813
2007	32.866
2008	32.919
2009	32.993
2010	33.089
2011	33.207
2012	33.346
2013	33.507
2014	33.689
2015	33.894
2016	34.121
2017	34.37