Year	Percent
1960	29.81468
1961	30.03625
1962	29.47549
1963	29.16756
1964	29.67041
1965	29.89551
1966	29.68963
1967	29.39182
1968	29.13043
1969	28.83243
1970	29.04501
1971	29.50311
1972	28.88355
1973	29.188
1974	28.75286
1975	28.64895
1976	28.16897
1977	28.11214
1978	28.01228
1979	27.62889
1980	27.76988
1981	27.06766
1982	26.44657
1983	26.24271
1984	25.7784
1985	25.63862
1986	25.45998
1987	25.05061
1988	24.98122
1989	24.38671
1990	24.45645
1991	24.2997
1992	24.00664
1993	23.06196
1994	22.24437
1995	22.30978
1996	21.67519
1997	21.5273
1998	21.44484
1999	21.20173
2000	21.29394
2001	20.93837
2002	20.64945
2003	20.19181
2004	19.95752
2005	19.60961
2006	19.28408
2007	19.13674
2008	18.81383
2009	18.30484
2010	17.84574
2011	17.97817
2012	17.86855
2013	17.9718
2014	17.74594
2015	17.67969
2016	17.56388
2017	17.36162