Year	Percent
1997	17.9
1998	16.7
1999	17.6
2000	16.4
2001	17.1
2002	15.1
2003	11.9
2004	13.6
2005	15.5
2006	13.5
2007	11.1
2008	8.9
2009	7
2010	5.5
2011	5.2
2012	4.7
2013	4.3
2014	3.7
2015	3.6
2016	3.5