Year	Percent
1970	37.2
1971	37.32
1972	36.31
1973	36.69
1974	37.11
1975	38.27
1976	37.08
1977	37.16
1978	37
1979	36.44
1980	35.93
1981	35.16
1982	33.96
1983	33.42
1984	32.62
1985	31.66
1986	31.8
1987	32.39
1988	32.57
1989	32.68
1990	33.5
1991	33.04
1992	32.8
1993	31.04
1994	30.26
1995	30.2
1996	29.47
1997	29.91
1998	30.51
1999	30.68
2000	30.83
2001	31.46
2002	31.04
2003	30.71
2004	30.21
2005	29.57
2006	29.4
2007	29.11
2008	27.83
2009	24.58
2010	22.97
2011	21.76
2012	20.67
2013	19.75
2014	19.45
2015	19.9
2016	19.61
2017	20.06