Year	Percent
1978	12.44
1979	11.15
1980	10.71
1981	10.42
1982	10.51
1983	11.07
1984	10.77
1985	10.81
1986	10.86
1987	9.9
1988	9.52
1989	9.19
1990	8.99
1991	9.12
1992	8.9
1993	8.5
1994	8.57
1995	8.49
1996	8.37
1997	7.97
1998	7.71
1999	7.74
2000	7.48
2001	7.25
2002	7.36
2003	7.26
2004	7.01
2005	6.7
2006	6.4
2007	6.22
2008	5.95
2009	5.72
2010	5.58
2011	5.39
2012	5.24
2013	5.17
2014	5.17
2015	5.13
2016	5.11
2017	5.09