Year	Percent
1968	0.5
1969	0.5
1970	0.5
1971	0.5
1972	1
1973	0.5
1974	0.5
1975	1.4
1976	0.9
1977	1.3
1978	1.3
1979	1.2
1980	1.2
1981	1.6
1982	1.5
1983	1.8
1984	2.1
1985	2
1986	2
1987	2.2
1988	2.2
1989	1.8
1990	1.4
1991	1.6
1992	1.3
1993	1.6
1994	1.9
1995	2.1
1996	2.6
1997	2.5
1998	2.9
1999	3.3
2000	3.6
2001	3.7
2002	3.8
2003	3.9
2004	3
2005	2.7
2006	2.8
2007	2.4
2008	2.6
2009	3.4
2010	3.3
2011	3.074
2012	3
2013	2.8
2014	2.5
2015	2.3
2016	2.3
2017	2.2