Year	Percent
1981	2.7
1982	2.6
1983	2.6
1984	2.7
1985	2.9
1986	2.7
1987	2.4
1988	2.1
1989	2
1990	2.3
1992	2.8
1993	2.4
1995	2.4
1996	2.2
1997	2.3
1998	2.4
1999	2.5
2001	2.5
2002	2.6
2003	2.6
2004	2.9
2005	2.9
2006	3
2007	3
2008	3.2
2009	3.2
2011	3.3
2012	3.4
2013	3.4
2014	3.7
2015	3.6