Year	Percent
1970	30.84
1971	32.78
1972	34.51
1973	36.16
1974	37.7
1975	38.77
1976	40.25
1977	41.48
1978	42.46
1979	43.78
1980	43.83
1981	43.82
1982	44.25
1983	44.24
1984	44.24
1985	44.54
1986	44.68
1987	44.83
1988	44.76
1989	45.09
1990	43.46
1991	39.9
1992	37.1
1993	35.82
1994	32.85
1995	30.95
1996	31.52
1997	30.3
1998	28.77
1999	27.06
2000	25.79
2001	25.85
2002	29.59
2003	29.14
2004	31.06
2005	30.45
2006	30.71
2007	31.41
2008	31.6
2009	30.02
2010	28.32
2011	28.56
2012	28.21
2013	28.28
2014	28.93
2015	28.46
2016	29.88
2017	30.12