Year	Percent
1960	19.27083
1961	17.71635
1962	19.94231
1963	20.83539
1964	18.49169
1965	20.37522
1966	21.16985
1967	20.96496
1968	26.85824
1969	26.17885
1970	24.35452
1971	24.40449
1972	24.01914
1973	28.31876
1974	26.7977
1975	32.21663
1976	29.88531
1977	29.64649
1978	30.10824
1979	27.39608
1980	22.28549
1981	22.95375
1982	29.40391
1983	24.03904
1984	25.81987
1985	22.84285
1986	25.70735
1987	23.99218
1988	23.13881
1989	23.15647
1990	23.92629
1991	26.96562
1992	29.0651
1993	28.38636
1994	27.19754
1995	27.18656
1996	27.27442
1997	29.35906
1998	32.58248
1999	27.28736
2000	27.32303
2001	25.25567
2002	27.71581
2003	26.3671
2004	26.32588
2005	32.0938
2006	32.76613
2007	34.26626
2008	37.96046
2009	32.90445
2010	34.32417
2011	38.38869
2012	37.78256
2013	37.13843
2014	41.97138
2015	37.06455
2016	31.91303
2017	31.80051