Year	Percent
1970	50.36
1971	50.54
1972	51.67
1973	49.59
1974	49.06
1975	47.92
1976	44.54
1977	41.91
1978	39.3
1979	37
1980	37.44
1981	38.19
1982	36.41
1983	33.87
1984	32.52
1985	30.58
1986	29.2
1987	27.37
1988	27.31
1989	25.56
1990	24.45
1991	22.73
1992	21.95
1993	23.15
1994	22.16
1995	21.08
1996	20.43
1997	19.62
1998	22.22
1999	20.11
2000	19.25
2001	18.09
2002	17.12
2003	16.64
2004	14.74
2005	13.97
2006	13.45
2007	12.68
2008	12.38
2009	11.82
2010	10.84
2011	10.63
2012	10.42
2013	10.03
2014	9.7
2015	9.03
2016	8.66
2017	8.38