Year	Percent
1991	20.1
1995	20.2
1996	20.2
1998	20.8
1999	21.3
2000	20.5
2001	20.9
2002	20.9
2003	21.6
2004	21.6
2005	21
2006	20.8
2007	20.5
2008	20.8
2009	20.5
2010	21.5
2011	21.3
2012	21.3
2013	20.7
2014	21.4
2015	21.4
2016	21.9