Year	Percent
1981	22.8
1986	24.9
1989	22.4
1990	22.5
1991	22.1
1992	22
1993	22.1
1994	21.4
1995	22
1996	21.9
1997	21.8
1998	21.7
1999	21.1
2000	21.5
2001	20.1
2002	19.7
2003	21.1
2004	21.1
2005	21
2006	20
2007	19.7
2008	20.3
2009	19.9
2010	20.4
2011	20.7
2012	20.6
2013	20.7
2014	20.8
2015	20.6
2016	20.6