Year	Percent
1998	40.1
1999	39.4
2000	39.1
2001	38.9
2002	38.8
2003	37.6
2004	35.9
2005	36.5
2006	37.1
2007	38.4
2008	36.8
2009	37
2010	37.5
2011	36.3
2012	36.1
2013	36.2
2014	36.7
2015	36.4
2016	36.9