Year	Percent
1970	16.23
1971	16.86
1972	17.7
1974	17.85
1975	17
1976	17.56
1977	16.79
1978	16.46
1979	17.32
1982	17.95
1983	17.53
1984	16.39
1985	17.45
1986	16.76
1987	17.2
1988	14.25
1989	14.69
1991	14.45
1992	16.98
1993	17.61
1994	18.36
1995	18.07
1996	18.37
1997	18.36
1998	18.07
1999	18.19
2000	17.36
2001	17.31
2002	16.99
2003	17.22
2004	17.64
2005	17.18
2006	18.07
2007	19.28
2008	19.08
2009	19.17
2010	18.7
2011	18.74
2012	18.25
2013	20.13
2014	20.08
2015	18.72
2016	18.39
2017	18.55