Year	Percent
1970	38.09
1971	37.31
1972	36.51
1973	36.53
1974	36.72
1975	36.14
1976	35.42
1977	34.33
1978	33.05
1979	32.51
1980	32.21
1981	31.31
1982	30.27
1983	29.88
1984	29.8
1985	29.84
1986	29.66
1987	29.91
1988	29.46
1989	29.47
1995	25.46
1996	25.84
1997	25.52
1998	25.86
1999	24.98
2000	24.36
2001	23.69
2002	22.93
2003	22.57
2004	22.35
2005	21.98
2006	21.85
2007	21.57
2008	21.23
2009	20.09
2010	19.85
2011	19.84
2012	19.54
2013	19.06
2014	18.5
2015	18.17
2016	18.08
2017	18.07