Year	Percent
1968	1.9
1969	1.7
1970	1.9
1971	1.9
1972	2.1
1973	2.2
1974	2.2
1975	2.6
1976	2.8
1977	3.1
1978	3.1
1979	3.2
1980	3.2
1981	3.8
1982	4.3
1983	4.2
1984	5
1985	4.7
1986	5.1
1987	5
1988	4.7
1989	4.2
1990	4.1
1991	4.2
1992	4.1
1993	5.1
1994	5.3
1995	6.1
1996	6.4
1997	6.6
1998	7.3
1999	8.2
2000	7.9
2001	8.4
2002	8.7
2003	8.6
2004	8.3
2005	7.4
2006	7.2
2007	7.1
2008	6.9
2009	8.4
2010	8
2011	7.087
2012	7.5
2013	6.2
2014	5.4
2015	5.1
2016	4.5
2017	4.5