Year	Percent
1978	23.68
1979	21.59
1980	21.36
1981	20.73
1982	20.73
1983	21.56
1984	20.85
1985	21
1986	20.54
1987	18.68
1988	18.44
1989	17.75
1990	17.57
1991	17.63
1992	17.1
1993	16.34
1994	16.25
1995	16.06
1996	15.9
1997	15.11
1998	14.6
1999	14.46
2000	13.92
2001	13.34
2002	13.44
2003	13.12
2004	12.54
2005	12
2006	11.44
2007	11.14
2008	10.38
2009	9.8
2010	9.54
2011	9
2012	8.62
2013	8.31
2014	8.24
2015	8.08
2016	8
2017	7.98