Year	Percent
1970	30.99139
1971	29.69879
1972	33.31938
1973	35.10466
1974	36.38462
1975	31.30637
1976	31.91205
1977	32.85796
1978	33.09088
1979	32.57732
1980	28.90545
1981	29.30519
1982	31.68646
1983	33.5775
1984	31.55135
1985	32.22409
1986	32.45553
1987	30.35198
1988	31.15421
1989	31.46093
1990	31.88455
1991	30.60625
1992	30.16088
1993	28.62844
1994	28.13335
1995	29.14276
1996	27.95489
1997	26.43511
1998	26.1334
1999	25.1439
2000	25.73902
2001	25.42067
2002	25.47357
2003	25.59649
2004	26.83965
2005	26.26922
2006	27.65225
2007	28.28979
2008	29.73664
2009	29.22869
2010	30.14394
2011	30.36911
2012	29.33422
2013	28.51819
2014	27.6255
2015	25.20242
2016	25.32636
2017	26.36997