Year	Percent
1986	21.4
1989	19.9
1992	18.9
1996	20.7
1997	20.6
2000	20
2001	19.8
2002	20.5
2003	19.3
2004	19.1
2005	19.9
2006	19.4
2007	19.9
2008	20.3
2009	19.9
2010	20.9
2011	20.5
2012	20.7
2013	20
2014	21.5
2015	21
2016	20.9