Year	Percent
1989	17.8
1990	18.8
1991	19.5
1992	19.2
1993	19.3
1994	19.2
1995	19
1996	19.4
1997	19.6
1998	19.3
1999	20.1
2001	20.6
2002	20
2003	19.3
2004	19.3
2005	19
2006	19.8
2007	19.5
2008	19.7
2009	21
2010	20.6
2011	19
2012	19.2
2013	20.3
2014	20.7
2015	21.4
2016	21.5