Year	Percent
1990	22
1997	18.8
1999	20.8
2000	18.8
2001	19.5
2002	18.9
2004	18.8
2005	19.2
2006	19.8
2007	19.8
2008	20.9
2009	21.3
2011	22.2
2012	22.5
2013	21.7
2014	21.3
2015	21.8
2016	22.8