Year	Percent
1960	14.97128
1961	15.38704
1962	15.81541
1963	16.2557
1964	16.70942
1965	17.17317
1966	17.64928
1967	18.0957
1968	18.5306
1969	18.97583
1970	19.43672
1971	19.91131
1972	20.39693
1973	20.88257
1974	21.36389
1975	21.83224
1976	22.29183
1977	22.61618
1978	22.85297
1979	23.05707
1980	23.22511
1981	23.36065
1982	23.48472
1983	23.61251
1984	23.76463
1985	23.95399
1986	24.1828
1987	23.97452
1988	23.69661
1989	23.52921
1990	23.49526
1991	23.60356
1992	23.65172
1993	23.70677
1994	23.76591
1995	23.78413
1996	23.74809
1997	23.74137
1998	23.74367
1999	23.76549
2000	23.83001
2001	23.93805
2002	24.08075
2003	24.2498
2004	24.43413
2005	24.6229
2006	24.81772
2007	24.99041
2008	25.15193
2009	25.30901
2010	25.46016
2011	25.60291
2012	25.65728
2013	25.67129
2014	25.69282
2015	25.72777
2016	25.77824
2017	25.84216