Year	Percent
1976	25.54
1977	26.15
1978	22.18
1981	21.56
1982	23.64
1983	23.41
1984	22.77
1985	23.24
1986	22.25
1987	22.37
1988	22.73
1989	23.18
1990	21.86
1991	20.12
1992	19.67
1993	19.08
1994	18.86
1995	19.56
1996	18.57
1997	19.2
1998	20.87
1999	21.41
2000	20.39
2001	19.63
2002	18.74
2003	17.61
2010	19.57
2011	20.33
2012	19.35
2013	18.97
2014	19.22
2015	19.34
2016	19.34