Year	Percent
1974	51.28
1975	46.79
1976	48.92
1977	50.18
1978	48.59
1979	48.39
1980	49.14
1981	48.47
1982	47.27
1983	46.21
1984	45.83
1985	46.8
1986	46.28
1987	45.54
1988	44.91
1989	44.57
1990	43.79
1991	43.25
1992	41.86
1993	41.62
1995	40.96
1996	40.82
1997	40.38
1998	40.62
1999	40.19
2001	41.22
2002	40.74
2003	41.16
2004	40.85
2005	39.35
2006	38.83
2007	37.75
2008	36.89
2009	34.84
2010	34.25
2015	32.56
2016	32.78