Year	Percent
1970	44.79
1971	43.79
1972	42.86
1973	42.44
1974	42.17
1975	39.86
1976	39.04
1977	38.9
1978	38.59
1979	38.22
1980	37.22
1981	35.35
1982	34.16
1983	35.67
1984	35
1985	34.07
1986	33.45
1987	32.57
1988	32.58
1989	32.41
1990	32.07
1991	30.93
1992	29.78
1993	29.21
1994	27.62
1995	27.3
1996	27.3
1997	26.77
1998	26.58
1999	25.74
2000	25.1
2001	24.57
2002	23.86
2003	23.3
2004	22.24
2005	22.16
2006	22
2007	22.11
2008	21.93
2009	19.48
2010	19.07
2011	19.04
2012	18.88
2013	18.69
2014	18.8
2015	18.53
2016	18.36
2017	18.13