Year	Percent
1974	34.51
1975	33.96
1976	33.51
1977	33.14
1978	35.29
1979	34.92
1980	36.43
1981	36.77
1982	37.28
1983	35.42
1984	33.81
1985	33.76
1986	33.77
1987	33.94
1988	34.52
1989	34.47
1990	33.88
1991	33.85
1992	32.43
1993	32.86
1994	32.47
1995	32.18
1996	31.27
1997	31.02
1998	35.21
1999	34.35
2000	34.44
2001	33.45
2002	33.33
2003	32.66
2004	31.06
2005	30.39
2006	30.31
2007	30.24
2008	28.99
2009	27.8
2010	27.26
2011	26.85
2012	25.15
2013	23.7
2014	23.86
2015	24.35
2016	24.5
2017	24.74