Year	Percent
1975	29.39
1976	30.09
1977	31.08
1978	32.02
1979	32.88
1980	33.39
1981	34.04
1982	34.46
1989	33.63
2005	32.09
2006	31.67
2007	32.21
2008	32.89
2009	31.67
2010	32.72
2011	33.49
2012	33.03
2013	33.26
2014	33.48
2015	32.83