Year	Percent
1970	22.28
1971	22.5
1972	22.69
1973	22.88
1974	23.23
1975	23.48
1976	23.67
1977	24.02
1978	24.22
1979	24.24
1980	24.54
1981	24.39
1982	24.66
1983	24.97
1985	73.03
1986	72.21
1987	72.3
1990	33.16
1991	40.64
1992	41.64
1994	38.69
1995	38.5
1996	39.4
1997	40.99
1998	42.44
1999	42.89
2001	40.87
2002	39.33
2003	41.23
2004	42.66
2005	43.27
2006	42.25
2007	43.46
2008	43.72
2009	43.52
2010	44.13
2011	44.86
2012	43.03
2013	46.38
2014	49.03
2015	49.45
2016	51.07
2017	47.33