Year	Percent
1997	12.2
1998	11.3
1999	11.3
2000	13.4
2001	12.7
2002	12.1
2003	11.9
2004	13
2005	12.9
2006	13.1
2007	13.2
2008	13.9
2009	13.9
2010	14.3
2011	14.7
2012	14.8
2013	14.8
2014	15
2015	14.8
2016	14.8