Year	Percent
1971	3.3
1972	3.7
1973	2.6
1974	2.6
1975	4
1976	5.5
1977	5.8
1978	5.7
1979	5.3
1980	6.8
1981	10.4
1982	10.9
1983	11.09
1984	10.9
1985	11.49
1986	11.51
1987	11.02
1988	9.01
1989	7.41
1990	6.97
1991	8.55
1992	9.78
1993	10.35
1994	9.65
1995	8.69
1996	8.19
1997	7.07
1998	6.2
1999	6.04
2000	5.56
2001	4.7
2002	5.04
2003	4.81
2004	4.59
2005	4.75
2006	5.35
2007	5.26
2008	5.62
2009	7.54
2010	7.79
2011	8.04
2012	7.88
2013	7.52
2014	6.11
2015	5.3
2016	4.81
2017	4.33
2018	4
2019	3.74