Year	Percent
1997	20.3
1998	18.8
1999	18.9
2000	21.3
2001	20.6
2002	19.6
2003	19.2
2004	20.8
2005	20.7
2006	21
2007	21
2008	21.7
2009	21.6
2010	21.7
2011	22
2012	21.9
2013	22
2014	22.2
2015	21.8
2016	22