Year	Percent
1970	31.29898
1971	33.0715
1972	31.75654
1973	32.72357
1974	33.12841
1975	26.86631
1976	36.38377
1977	31.20651
1978	28.39971
1979	28.54866
1980	39.07776
1981	34.81024
1982	38.83147
1983	46.10288
1984	44.31139
1985	37.87201
1986	34.94744
1987	27.14557
1988	25.41271
1989	31.96927
1990	19.28591
1991	34.23516
1992	38.67476
1993	33.17169
1994	30.87744
1995	30.80838
1996	29.02413
1997	29.57032
1998	24.55583
1999	26.65126
2000	41.80038
2001	46.06597
2002	38.81651
2003	38.26753
2004	39.38741
2005	38.97957
2006	41.39375
2007	41.63027
2008	38.28258
2009	40.7438
2010	39.36506
2011	40.56143
2012	38.06588
2013	40.10979
2014	41.92935
2015	36.42825
2016	36.35287
2017	32.34389