Year	Percent
1990	22.3
1995	18.8
1997	20.4
1999	19.7
2001	19.1
2002	18.3
2003	18.7
2004	19
2005	19.7
2006	19.2
2007	18.9
2008	19.8
2009	20.5
2010	19.7
2011	19.6
2012	20.7
2013	20.8
2014	19.4
2015	20.9
2016	20.6