Year	Percent
1970	30.11
1973	31.73
1974	35.03
1975	32.31
1976	33.23
1977	33.24
1978	34.71
1979	35.25
1980	35.73
1981	37.12
1982	36.67
1983	35.9
1984	36.86
1985	35.23
1986	34.7
1987	35.05
1988	35.89
1989	36.18
1990	37.93
1991	35.17
1992	34.63
1993	33.88
1994	32.74
1995	30.96
1996	30.25
1997	30.21
1998	29.54
1999	28.86
2000	33.81
2001	25.77
2002	25.13
2003	25
2004	23.95
2005	21.66
2006	23.33
2007	23.62
2008	23.76
2010	30.38
2011	19.62
2012	19.24
2013	18.57
2014	16.52
2015	16.25
2016	27.32