Year	Percent
1987	13.4
1988	12.8
1989	15
1990	14.2
1991	15.06
1992	12.78
1993	14.03
1995	18.73
1996	15.85
1997	15.14
1998	17.48
1999	14.18
2000	13.78
2001	12.45
2002	11.27
2003	11.53
2004	10.63
2005	10.81
2006	9.67
2007	9.61
2008	9.49
2009	8.84
2010	8.94
2011	8.43
2012	8.65
2013	9.11
2014	9.49
2015	9.11
2016	8.81