Year	Percent
1960	19.20242
1961	19.51604
1962	19.87212
1963	20.2415
1964	20.62515
1965	21.01955
1966	21.3941
1967	21.42635
1968	21.46935
1969	21.52695
1970	21.60485
1971	21.70275
1972	21.81742
1973	21.93939
1974	22.06214
1975	22.17823
1976	22.28691
1977	22.34562
1978	22.36888
1979	22.3809
1980	22.38189
1981	22.3718
1982	22.35407
1983	22.32609
1984	22.2863
1985	22.2329
1986	22.16743
1987	22.18896
1988	22.27902
1989	22.39553
1990	22.55278
1991	22.75285
1992	22.99227
1993	23.26188
1994	23.5556
1995	23.86546
1996	24.19192
1997	24.3554
1998	24.42487
1999	24.49493
2000	24.56326
2001	24.62421
2002	24.68016
2003	24.73539
2004	24.79759
2005	24.86827
2006	24.95304
2007	25.04678
2008	25.13649
2009	25.20533
2010	25.24336
2011	25.24771
2012	25.22686
2013	25.196
2014	25.1746
2015	25.17583
2016	25.20264
2017	25.25147