Year	Percent
1972	34.24
1973	33.78
1974	32.29
1975	31.47
1976	31.32
1977	30.43
1978	30.34
1979	32.52
1981	29.36
1983	27.6
1984	26.58
1985	27.79
1986	27.87
1987	27.83
1988	26.95
1989	27.17
1990	27.29
1991	27.45
1992	27.02
1993	25.99
1994	26.39
1995	27.03
1996	26.35
1997	26.13
1998	26.38
1999	26.83
2000	25.25
2001	25.34
2002	23.43
2003	23.1
2004	23.64
2005	23.85
2006	23.3
2007	23.48
2008	23.1
2009	20.28
2010	19.6
2011	19.87
2012	19.68
2013	19.43
2014	19.21
2015	19.23
2016	18.46
2017	18.62