Year	Percent
1973	16.7
1974	18.47
1975	18.46
1976	18.46
1977	18.46
1978	18.46
1979	20.32
1980	20.32
1981	20.32
1982	20.32
1983	19.47
1984	19.47
1985	20.13
1986	19.16
1987	20.97
1988	19.82
1989	19.82
1990	19.81
1991	19.83
1992	19.65
1993	18.78
1994	17.49
1995	18.52
1996	18.53
1997	18.92
1998	17.11
1999	17.11
2000	18.03
2001	18.03
2002	20.76
2003	20.76
2004	20.3
2005	20.3
2006	20.72
2007	20.97
2008	20.1
2009	20.98
2010	21.42
2011	21.77
2013	22.92
2014	22.87
2015	24.02
2016	23.59