Year	Percent
1970	30.05
1971	29.96
1972	29.86
1973	30.72
1974	33.55
1975	32.13
1977	35.31
1978	34.64
1979	34.96
1980	35.68
1981	35.97
1982	37.56
1983	36.15
1984	36.21
1989	36.79
1990	40.08
1991	43.81
1992	40.05
1993	43.02
1994	43.32
1995	44.11
1996	44.59
1997	46.16
1998	47.94
1999	48.72
2000	49.06
2001	50.18
2002	51.91
2003	50.37
2004	48.2
2005	47.47
2006	46.56
2007	46.01
2008	45.29
2009	46.24
2010	46.27
2011	47.08
2012	48.06
2013	47.93
2014	48.04
2015	49.1
2016	48.84
2017	48.37