Year	Percent
1970	36.36
1971	36.76
1972	35.21
1973	35.52
1974	35.16
1975	33.53
1976	33.2
1977	32.43
1978	31.23
1979	31.22
1980	30.96
1981	30.52
1982	29.74
1983	28.41
1984	27.98
1985	26.96
1986	26.25
1987	25.84
1988	25.65
1989	25.85
1990	25.06
1991	23.83
1992	23.58
1993	23.61
1994	23.47
1995	22.86
1996	22.46
1997	22.15
1998	21.8
1999	21.37
2000	21.62
2001	20.74
2002	20.91
2003	20.88
2004	21.24
2005	21.13
2006	21.44
2007	21.36
2008	21.62
2009	21.14
2010	20.98
2011	20.83
2012	20.7
2013	20.45
2014	21.04
2015	20.14
2016	20.07
2017	19.43