Year	Percent
1977	37.83
1978	37.48
1979	37.15
1980	37.24
1981	36.9
1982	36.42
1983	36
1984	34.61
1985	33.53
1986	33.15
1987	32.24
1988	32.2
1989	31.97
1990	32.39
1991	32.2
1992	34.63
1993	34.1
1994	33.76
1995	33.66
1996	33.35
1997	32.62
1998	32.73
1999	32.38
2000	31.79
2001	31.74
2002	31.59
2003	31.77
2004	30.84
2005	30.68
2006	30.05
2007	30.13
2008	29.8
2009	29.24
2010	28.61
2011	28.29
2012	27.58
2013	27.05
2014	26.9
2015	26.6
2016	26.12
2017	26