Year	Percent
1970	4.5
1971	5.1
1972	5.3
1973	5.1
1974	4.9
1975	8.1
1976	8.2059
1977	9.1707
1978	9.5466
1979	8.6924
1980	8.3288
1981	8.2561
1982	10.7433
1983	11.5626
1984	11.3092
1985	10.5692
1986	9.7568
1987	9.2043
1988	8.1902
1989	7.7674
1990	8.0913
1991	9.69
1992	10.1922
1993	10.6769
1994	9.7962
1995	9.1147
1996	9.2883
1997	8.9185
1998	7.9867
1999	7.2933
2000	6.6947
2001	6.8562
2002	7.1249
2003	7.1577
2004	6.8529
2005	6.4647
2006	6.0925
2007	5.6495
2008	5.6515
2009	7.0123
2010	7.237
2011	7.0092
2012	6.8219
2013	6.5973
2014	6.3904
2015	6.2859
2016	6.2401
2017	5.8502