Year	Percent
1960	19.58765
1961	20.77426
1962	22.01575
1963	23.30545
1964	24.64065
1965	26.01
1966	26.58417
1967	26.80192
1968	27.00664
1969	27.21157
1970	27.43401
1971	27.678
1972	27.94377
1973	28.2229
1974	28.51449
1975	28.8143
1976	29.12455
1977	29.44599
1978	29.47044
1979	29.38719
1980	29.33997
1981	29.32153
1982	29.33146
1983	29.37289
1984	29.45204
1985	29.56567
1986	29.71946
1987	29.9021
1988	30.08834
1989	30.24301
1990	30.3482
1991	30.39908
1992	30.40684
1993	30.37803
1994	30.33066
1995	30.27512
1996	30.21386
1997	30.14579
1998	30.01484
1999	29.87481
2000	29.76448
2001	29.67921
2002	29.61755
2003	29.57825
2004	28.39357
2005	27.28973
2006	26.26439
2007	25.30741
2008	25.62823
2009	25.9239
2010	25.80877
2011	25.63695
2012	25.42482
2013	25.19652
2014	24.98645
2015	24.81321
2016	24.68068
2017	24.57809