Year	Percent
1968	1.2
1969	1.2
1970	1.3
1971	1.5
1972	1.7
1973	1.5
1974	1.6
1975	2.3
1976	2.6
1977	2.8
1978	3
1979	3.1
1980	2.8
1981	3.2
1982	3.4
1983	3.7
1984	3.9
1985	3.7
1986	4.1
1987	4.2
1988	3.6
1989	3.1
1990	2.7
1991	2.5
1992	2.5
1993	3
1994	3.6
1995	3.7
1996	4.2
1997	4
1998	5
1999	5.4
2000	5.55
2001	5.77
2002	5.88
2003	5.7
2004	4.53
2005	4.15
2006	3.91
2007	3.4
2008	3.72
2009	4.77
2010	5.01
2011	4.46
2012	4.12
2013	3.7
2014	3.2
2015	3.1
2016	2.9
2017	2.7
2018	2.3
2019	2.1
2020	2.6