Year	Percent
1970	14.49
1971	14.7
1972	14.88
1973	14.99
1974	15.08
1975	15.19
1976	15.39
1977	15.49
1978	15.68
1979	15.7
1980	16.86
1981	16.89
1982	17.02
1983	17.25
1985	25.9
1986	23.96
1987	23.9
1990	16.67
1991	21.22
1992	20.11
1994	24.04
1995	24.24
1996	22.3
1997	22.01
1998	22.97
1999	22.04
2001	22.17
2002	21.81
2003	21.41
2004	22.36
2005	23.46
2006	22.09
2007	22.21
2008	22
2009	20.74
2010	18.93
2011	19.74
2012	19.91
2013	19.01
2014	21.98
2015	21.81
2016	21.57
2017	20.68