Year	Percent
1968	0.6
1969	0.6
1970	0.6
1971	1.3
1972	1.3
1973	1.2
1974	1.2
1975	1.8
1976	2.4
1977	2.3
1978	2.3
1979	2.2
1980	2.2
1981	2.6
1982	2.2
1983	2.7
1984	2.2
1985	2.1
1986	1.6
1987	1.6
1988	1.5
1989	1.5
1990	1.4
1991	1.3
1992	1.6
1993	1.6
1994	1.9
1995	2.2
1996	2.1
1997	2
1998	2.6
1999	2.9
2000	3.2
2001	3.2
2002	2.9
2003	3.3
2004	2.6
2005	2.5
2006	2.8
2007	2.3
2008	2.5
2009	3.3
2010	3.3
2011	2.924
2012	2.9
2013	2.8
2014	2.6
2015	2.4
2016	2.5
2017	2.2