Year	Percent
1971	2.1
1972	2.7
1973	2.8
1974	3
1975	5.1
1976	6.5
1977	5.67
1978	7.9
1979	6.96
1980	9.1
1981	11.35
1982	13
1983	11.66
1984	11.88
1985	11.32
1986	11.28
1987	11.26
1988	10.13
1989	8.28
1990	7.25
1991	6.99
1992	6.7
1993	8.08
1994	9.65
1995	9.34
1996	9.48
1997	8.96
1998	9.32
1999	8.65
2000	6.59
2001	6.18
2002	6.91
2003	7.68
2004	7.36
2005	8.44
2006	8.25
2007	7.46
2008	6.98
2009	7.91
2010	8.29
2011	7.14
2012	7.54
2013	8.43
2014	8.52
2015	8.48
2016	7.83
2017	7.09
2018	5.95
2019	5.36
2020	5.55