Year	Percent
1981	10.5
1982	10.3
1983	10
1984	10.2
1985	11
1986	10.3
1987	10.1
1988	9.6
1989	8.9
1990	9.7
1992	12.2
1993	10.1
1995	10.1
1996	10
1997	10
1998	10.1
1999	10.2
2001	10.5
2002	10.5
2003	10.7
2004	11
2005	11.1
2006	11.2
2007	11.6
2008	11.8
2009	12
2011	12.2
2012	12.3
2013	12.3
2014	12.5
2015	12.7