Year	Percent
1975	31.58
1976	32.91
1977	32.52
1978	33.62
1979	32.89
1980	33.97
1985	29.34
1986	29.01
1987	29.99
1988	29.98
1989	29.89
1990	30.88
1991	30.6
1992	30.9
1993	16.5
1994	31.42
1995	31.5
1996	29.78
1997	27.59
1998	26.5
1999	24.07
2000	25.54
2001	18.43
2009	19.81
2010	19.99
2011	20.66
2012	20.66
2013	19.63
2014	19.78
2015	19.82
2016	19.41
2017	19.41