Year	Percent
1960	14.08605
1961	14.18827
1962	14.26952
1963	14.33072
1964	14.37593
1965	14.40691
1966	14.44867
1967	14.60394
1968	14.75218
1969	14.8972
1970	15.04475
1971	15.19495
1972	15.34715
1973	15.49832
1974	15.64892
1975	15.79658
1976	15.94427
1977	16.08938
1978	16.22681
1979	16.34673
1980	16.44517
1981	16.51843
1982	16.57509
1983	16.62705
1984	16.70558
1985	16.84759
1986	17.01728
1987	17.21148
1988	17.42583
1989	17.64968
1990	17.88098
1991	18.11766
1992	18.36581
1993	18.63244
1994	18.83093
1995	18.65367
1996	18.5144
1997	18.40741
1998	18.32944
1999	18.27521
2000	18.2402
2001	18.22327
2002	18.22252
2003	18.22893
2004	18.25742
2005	18.39714
2006	18.52369
2007	18.63768
2008	18.74144
2009	18.83764
2010	18.93058
2011	19.02025
2012	19.10677
2013	19.19068
2014	19.27547
2015	19.36256
2016	19.45289
2017	19.54737