Year	Percent
1998	51.1
2000	40.2
2001	39.1
2002	39.1
2003	38.1
2004	43
2005	41.3
2006	44.9
2007	41.8
2008	40
2009	39.3
2010	39.2
2011	37.7
2012	37.3
2013	38.3
2014	37
2015	38.8
2016	37.2