Year	Percent
1960	19.17469
1961	22.58068
1962	18.96328
1963	19.94361
1964	24.63457
1965	25.89921
1966	25.56289
1967	24.29936
1968	24.53456
1969	23.55714
1970	24.10086
1971	25.0214
1972	24.89224
1973	26.29415
1974	32.88737
1975	25.54767
1976	24.05281
1977	26.09081
1978	25.36641
1979	29.53369
1980	32.77676
1981	35.24202
1982	35.0751
1983	34.37286
1984	35.22907
1985	32.27091
1986	28.02242
1987	32.23412
1988	35.55921
1989	35.40022
1990	35.88288
1991	33.81571
1992	31.84547
1993	30.57208
1994	32.04064
1995	33.14148
1996	33.66142
1997	35.1419
1998	36.48885
1999	36.82132
2000	39.23253
2001	37.64007
2002	35.2155
2003	33.68656
2004	35.50859
2005	35.83755
2006	38.15597
2007	38.64734
2008	40.83806
2009	34.00157
2010	37.4316
2011	38.57255
2012	38.21839
2013	38.51117
2014	39.88538
2015	39.82035
2016	39.00844
2017	42.0834