Year	Percent
1968	1.6
1969	1.5
1970	1.8
1971	2.1
1972	2.4
1973	1.8
1974	2.1
1975	3.2
1976	3.8
1977	3.8
1978	4.2
1979	4.4
1980	4
1981	4.3
1982	4.6
1983	5
1984	5.2
1985	5
1986	5.2
1987	5.4
1988	4.4
1989	3.9
1990	3.4
1991	3
1992	3.2
1993	3.8
1994	4.6
1995	4.6
1996	5.1
1997	5
1998	6.3
1999	6.7
2000	6.7
2001	7
2002	7.1
2003	6.8
2004	5.5
2005	5
2006	4.5
2007	4.1
2008	4.3
2009	5.6
2010	6
2011	5.335
2012	4.9
2013	4.4
2014	3.7
2015	3.7
2016	3.4
2017	3