Year	Percent
1960	31.80026
1961	36.60376
1962	29.84085
1963	34.76699
1964	32.02524
1965	29.92623
1966	29.80272
1967	29.15541
1968	30.68902
1969	31.31218
1970	32.18369
1971	32.72214
1972	32.74114
1973	33.42209
1974	34.81859
1975	35.47128
1976	34.97984
1977	34.0516
1978	35.52061
1979	36.51959
1980	39.59242
1981	39.24266
1982	40.98651
1983	39.48474
1984	41.67583
1985	41.24053
1986	40.1932
1987	41.29284
1988	39.44697
1989	42.28383
1990	32.83062
1991	31.51262
1992	33.9876
1993	36.12568
1994	34.66694
1995	23.38146
1996	22.34011
1997	22.59415
1998	22.11879
1999	21.74671
2000	23.00662
2001	22.63953
2002	22.49533
2003	23.08382
2004	24.30662
2005	24.17258
2006	23.54402
2007	23.12538
2008	23.08529
2009	21.87859
2010	23.26798
2011	23.10205
2012	22.13365
2013	21.22481
2014	20.47246
2015	19.35979
2016	18.27879
2017	18.47557