Year	Percent
1970	33.89
1971	34.79
1973	33.44
1974	33.18
1975	33.87
1977	36.08
1978	37.55
1979	38.58
1980	36.92
1981	39.55
1982	39.14
1983	37.78
1984	35.5
1985	33.04
1986	29.32
1987	29.06
1988	27.73
1989	27.1
1990	27.16
1991	28.75
1992	26.95
1993	26.38
1994	25.41
1995	25.52
1996	25.53
1997	26.38
1998	28.19
1999	27.71
2000	28.05
2001	28.79
2002	28.43
2003	28.46
2004	30.22
2005	30.96
2006	30.76
2007	31.83
2008	32.23
2011	28.47
2012	28.44
2013	28.59
2014	27.94
2015	27.83
2016	27.31