Year	Percent
1983	36.01
1984	35.73
1985	33.89
1986	35.7
1987	33.89
1988	32.14
1989	31.29
1990	27.66
1991	25.17
1992	25.97
1993	25.18
1994	25.64
1995	25.11
1996	24.25
1997	23.61
1998	21.29
1999	18.46
2000	17.2
2001	14.76
2002	13.92
2003	13.65
2004	11.31
2005	11.13
2006	10.8
2007	10.48
2008	9.68
2009	9.55
2010	9.12
2011	8.48
2012	7.77
2013	7.72
2014	7.07
2015	6.3
2016	5.77
2017	5.96