Year	Percent
1970	38.69
1971	38.73
1972	38.78
1973	38.95
1974	38.87
1975	38.06
1976	37.45
1977	37
1978	36.31
1979	35.8
1980	35.47
1981	34.72
1982	34.06
1983	33.28
1984	32.4
1985	31.55
1986	30.94
1987	30.37
1988	29.9
1989	29.73
1990	29.59
1991	29.14
1992	29.33
1993	27.47
1994	26.81
1995	26.93
1996	26.5
1997	26.55
1998	26.32
1999	26.29
2000	26.26
2001	26.03
2002	25.39
2003	23.95
2004	24.18
2005	23.69
2006	23.76
2007	23.29
2008	23.13
2009	22.56
2010	22.15
2011	22.08
2012	21.66
2013	21.13
2014	20.32
2015	20.12
2016	20.03
2017	20.24