Year	Percent
1980	41.11
1981	38.52
1982	37.78
1983	37.22
1984	36.95
1985	36.9
1986	36.7
1987	36.43
1988	35.97
1989	35.36
1990	36.79
1991	36.07
1992	35.05
1993	33.76
1994	32.98
1995	32.57
1996	33.24
1997	33.23
1998	34.78
1999	34.35
2000	33.77
2001	34.42
2002	34.14
2003	33.4
2004	32.95
2005	32.48
2006	32.32
2007	32.51
2008	32.22
2009	31.16
2010	30.7
2011	30.84
2012	29.79
2013	29.9
2014	30.39
2015	30.28
2016	30.41
2017	31.52