Year	Percent
1970	26.89
1971	26.42
1972	27.2
1973	27.5
1974	26.02
1975	25.08
1976	24.29
1977	21.95
1978	20.33
1979	18.77
1980	18.78
1981	19.15
1982	18.3
1983	16.81
1984	15.39
1985	14.61
1986	14.21
1987	13.26
1988	12.85
1989	12.07
1990	11.43
1991	10.58
1992	10.14
1993	10.57
1994	10.05
1995	9.53
1996	9.32
1997	9
1998	10.16
1999	9.49
2000	9.12
2001	8.62
2002	8.09
2003	7.62
2004	6.85
2005	6.56
2006	6.32
2007	6
2008	5.89
2009	5.66
2010	5.27
2011	5.11
2012	5.01
2013	4.83
2014	4.65
2015	4.37
2016	4.24
2017	4.13