Year	Percent
1968	1.8
1969	1.9
1970	2
1971	2
1972	2.5
1973	2.2
1974	2.4
1975	3.1
1976	3.1
1977	3.5
1978	3.8
1979	3.7
1980	3.6
1981	3.9
1982	4.4
1983	4.5
1984	4.9
1985	4.8
1986	5.2
1987	5.2
1988	4.9
1989	4.6
1990	4.3
1991	4.5
1992	4.4
1993	5
1994	5.6
1995	6.1
1996	6.6
1997	6.7
1998	7.7
1999	9.1
2000	9.19
2001	9.58
2002	10.04
2003	10.17
2004	9.54
2005	8.68
2006	8.27
2007	7.84
2008	7.29
2009	9.18
2010	9.43
2011	8.3
2012	8.22
2013	6.9
2014	6.3
2015	5.5
2016	5.1
2017	4.6
2018	3.6
2019	3.8
2020	4.6