Year	Percent
1960	17.08293
1961	17.62639
1962	17.61023
1963	17.8641
1964	19.15926
1965	23.04481
1966	22.80766
1967	23.07083
1968	23.78364
1969	24.3094
1970	24.49549
1971	23.0478
1972	24.58801
1973	27.32203
1974	26.31632
1975	26.36814
1976	27.28471
1977	28.08648
1978	29.80766
1979	31.14569
1980	31.49858
1981	30.67756
1982	31.00921
1983	32.3787
1984	33.89711
1985	33.39183
1986	34.2519
1987	35.40114
1988	36.01248
1989	35.57064
1990	35.77172
1991	36.48938
1992	35.21767
1993	35.51856
1994	35.58984
1995	35.80317
1996	34.80703
1997	34.72388
1998	34.71837
1999	34.05503
2000	34.19664
2001	32.94196
2002	32.42908
2003	32.74294
2004	34.24545
2005	33.83844
2006	33.23619
2007	33.4133
2008	32.72239
2009	33.26623
2010	34.63472
2011	34.84625
2012	34.58738
2013	35.02176
2014	34.70009
2015	34.89109
2016	35.1298
2017	35.86926