Year	Percent
1987	20.4
1994	21.6
2002	21.6
2003	21.5
2004	22.4
2005	21.7
2006	22.5
2007	22.7
2008	22.4
2009	22.3
2010	22.3
2011	21.9
2012	22
2013	21.9
2014	21.6
2015	21.1
2016	21.6