Year	Percent
1970	42.16
1971	41.86
1972	41.11
1973	40.88
1974	40.6
1975	39.31
1976	38.33
1977	37.37
1978	36.06
1979	35.03
1980	34.31
1981	32.85
1982	31.78
1983	33.48
1984	32.6
1985	31.86
1986	31.99
1987	31.61
1988	31.36
1989	31.42
1990	30.7
1991	30.51
1992	30.62
1993	29.46
1994	28.88
1995	28.25
1996	27.63
1997	27.55
1998	27.25
1999	25.77
2000	25.81
2001	25.49
2002	25.78
2003	24.92
2004	24.94
2005	24.69
2006	24.69
2007	24.42
2008	24.67
2009	23.47
2010	23.38
2011	23.23
2012	21.76
2013	21.7
2014	21.39
2015	21.43
2016	21.26
2017	20.76