Year	Percent
1981	33.7
1986	23.1
1989	33.7
1990	33
1991	34.3
1992	33.7
1993	33.7
1994	35.3
1995	33.8
1996	34.3
1997	33.8
1998	34
1999	35.9
2000	35.7
2001	39.8
2002	40.5
2003	37
2004	36.5
2005	35.8
2006	38.5
2007	38.9
2008	37.7
2009	39.4
2010	37.1
2011	37.1
2012	37
2013	37.4
2014	36.9
2015	36.8
2016	37.3