Year	Percent
1993	23.6
1996	22.7
2004	23.1
2005	22.9
2006	22.6
2007	22.3
2008	22.9
2009	22.3
2010	22.4
2011	22.2
2012	22.1
2013	22.6
2014	22.1
2015	22.1