Year	Percent
1970	45.87
1971	45.85
1972	45.4
1973	44.85
1974	44.31
1975	42.23
1976	40.85
1977	40.45
1978	40.15
1979	38.16
1980	38.12
1981	37.86
1982	36.92
1983	36.01
1984	35.71
1985	35.57
1986	35.28
1987	34.42
1988	33.57
1989	32.96
1990	32.16
1991	28.34
1992	27.32
1993	26.51
1994	26.65
1995	27.69
1996	25.45
1997	24.5
1998	24.08
1999	23.76
2000	24.05
2001	24.33
2002	23.11
2003	22.26
2004	22.15
2005	22.34
2006	22.51
2007	22.39
2008	21.12
2009	20.95
2010	21.36
2011	21.31
2012	20.78
2013	21.04
2014	20.61
2015	20.2
2016	20.05
2017	20.02