Year	Percent
1972	34.2
1973	33.86
1974	34.18
1975	34.39
1976	33.15
1977	32.35
1978	31.61
1979	30.13
1980	29.19
1981	28.84
1982	28.1
1983	26.89
1984	27.16
1985	26.81
1986	26.65
1987	26.58
1988	25.92
1989	24.89
1990	24.33
1991	23.18
1992	23.05
1993	22.7
1994	23
1995	22.99
1996	22.99
1997	23.43
1998	22.79
1999	22.25
2000	21.77
2001	21.73
2002	21.65
2003	21.64
2004	20.78
2005	20.84
2006	20.81
2007	21.17
2008	20.55
2009	20.2
2010	19.68
2011	20.23
2012	20.2
2013	20.26
2014	20.38
2015	20.08
2016	19.4
2017	19.36