Year	Percent
1987	22.3
1989	22.1
1993	21.5
2004	22.3
2005	21.6
2006	22.6
2007	22.8
2008	22.8
2009	23
2010	22.9
2011	22.7
2012	22.8
2013	22.7
2014	22.9
2015	22.9