Year	Percent
1970	38.91
1971	38.05
1972	37
1973	36.47
1974	35.81
1975	34.9
1976	33.58
1977	34.71
1978	32.98
1979	32.9
1980	31.45
1981	30.75
1982	28.74
1983	28.72
1984	28.27
1985	28.01
1986	26.79
1987	26.72
1988	26.44
1989	26.56
1990	26.23
1991	25.4
1992	23.62
1993	23.58
1994	22.85
1995	22.64
1996	21.83
1997	21.54
1998	21.68
1999	21.17
2000	20.21
2001	20.38
2002	19.01
2003	19.23
2004	19.12
2005	19.56
2006	19.3
2007	18.53
2008	17.62
2009	16.58
2010	15.95
2011	15.32
2012	15.48
2013	15.14
2014	14.96
2015	15.16
2016	15.14
2017	14.9