Year	Percent
1997	41.2
1998	44.5
1999	44.9
2000	37.1
2001	39.3
2002	42.3
2003	42.6
2004	38.4
2005	38.8
2006	38.3
2007	38.2
2008	35.8
2009	35.5
2010	34.4
2011	33.7
2012	33.5
2013	32.9
2014	32.3
2015	32.8
2016	32.7