Year	Percent
1970	44.75
1971	44.08
1972	43.7
1973	45.11
1974	45.57
1975	43.43
1976	41.83
1977	40.78
1978	39.22
1979	38.45
1980	38.05
1981	37.11
1982	36.2
1983	31.93
1984	31.96
1985	31.64
1986	29.58
1987	31.06
1988	29.21
1989	29.19
1990	29.41
1991	28.98
1992	28.61
1993	26.28
1994	26.84
1995	25.11
1996	22.9
1997	23.29
1998	21.52
1999	21.9
2000	20.75
2001	21.36
2002	20.06
2003	19.23
2004	18.8
2005	17.27
2006	16.68
2007	16.91
2008	15.26
2009	12.6
2010	12.66
2011	12.95
2012	12.4
2013	11.71
2014	10.75
2015	11.43
2016	10.72
2017	8.96