Year	Percent
1960	34.18181
1961	35.76159
1962	37.70491
1963	38.48581
1964	40.80459
1965	37.66938
1966	42.52577
1967	46.89824
1968	39.63123
1969	37.59846
1970	41.56235
1971	37.42346
1972	33.81309
1973	36.48688
1974	43.33611
1975	39.91997
1976	29.32661
1977	34.32491
1978	33.88576
1979	35.13294
1980	41.08078
1981	34.05974
1982	34.22451
1983	36.61626
1984	35.50168
1985	31.63598
1986	28.62263
1987	28.55029
1988	25.05388
1989	26.46316
1990	25.87551
1991	26.12265
1992	26.36338
1993	22.88907
1994	25.01862
1995	28.25495
1996	25.61402
1997	28.93423
1998	28.39265
1999	23.98869
2000	25.28757
2001	22.57325
2002	20.14586
2003	17.99522
2004	19.24604
2005	21.70818
2006	22.07987
2007	23.46557
2008	23.43386
2009	23.2356
2010	26.50979
2011	22.02757
2012	21.7821
2013	25.00494
2014	37.56252
2015	34.5787
2016	30.02529
2017	31.7748