Year	Percent
1990	15.9
1991	14.66
1992	14.22
1993	13.82
1994	13.03
1995	12.27
1996	11.35
1997	10.6
1998	9.17
1999	8.88
2000	7.74
2001	7.9
2002	8.76
2003	8.22
2004	8.38
2005	7.67
2006	6.5
2007	5.97
2008	5.22
2009	5.85
2010	4.78
2011	4.12
2012	3.88
2013	4.24
2014	4.19
2015	4.52
2016	4.24
2017	4.05
2018	4.32