Year	Percent
1976	28.15
1977	31.14
1978	32.77
1979	33.46
1980	33.82
1981	33.08
1982	32.54
1983	31.77
1984	31.57
1985	30.97
1986	30.34
1987	29.78
1988	29.65
1989	28.98
1990	28.65
1991	28.91
1992	28.11
1993	26.8
1994	25.85
1995	25.25
2000	23.77
2001	23.69
2002	23.18
2003	22.88
2004	23.38
2005	24.05
2006	22.65
2007	22.52
2008	23.05
2009	21.82
2010	20.4
2011	20.9
2012	20.17
2013	17.56
2014	16.52
2015	16.23
2016	17.05
2017	17.05