Year	Percent
1977	23.98
1978	25.58
1980	23.48
1983	20.51
1984	20.84
1985	20.7
1986	20.81
1987	21.75
1989	21.45
1990	21.92
1991	22.91
1992	22.2
1993	21.93
1994	21.26
1995	21.46
1996	20.74
1997	19.78
1998	18.54
2000	18.01
2002	17.52
2003	16.66
2004	15.92
2005	15.3
2006	15.84
2007	16.17
2008	17.6
2009	16.01
2011	15.33
2013	15.86