Year	Percent
1987	20.9
1994	19.1
1998	20.7
1999	17.7
2000	18.1
2003	19.5
2004	19.4
2005	19.9
2006	19.6
2007	19.4
2008	20.6
2009	20.8
2010	20.5
2011	21.7
2012	21.6
2013	21
2014	21.2
2015	21.3
2016	21.4