Year	Percent
1960	33.6701
1961	31.12995
1962	30.12515
1963	28.96276
1964	29.21223
1965	30.69586
1966	30.8728
1967	29.37301
1968	29.29736
1969	28.0887
1970	30.66392
1971	35.18911
1972	28.72708
1973	28.66901
1974	40.89752
1975	34.51158
1976	31.75564
1977	31.5931
1978	38.68802
1979	31.61102
1980	35.89983
1981	33.8203
1982	31.55827
1983	28.21277
1984	32.054
1985	30.1465
1986	29.89303
1987	26.39755
1988	27.60377
1989	30.12335
1990	31.32831
1991	28.55607
1992	26.6705
1993	33.95485
1994	34.22584
1995	39.15404
1996	32.11151
1997	31.37073
1998	28.72798
1999	27.35954
2000	31.72147
2001	33.01526
2002	30.2747
2003	30.04545
2004	32.86674
2005	35.96984
2006	32.25155
2007	31.9758
2008	34.90454
2009	30.83101
2010	33.56988
2011	38.8213
2012	35.59283
2013	33.2076
2014	33.00132
2015	27.61888
2016	22.78015
2017	25.4687