Year	Percent
1979	42.55
1980	42.71
1981	43.22
1982	43.83
1983	44.4
1984	44.79
1985	45.04
1986	45.36
1987	45.8
1988	45.7
1989	45.34
1990	44.19
1991	41.12
1992	38.55
1993	36.59
1994	34.81
1995	33.5
1996	32.58
1997	32
1998	30.63
1999	28.86
2000	32.66
2001	32.72
2002	32.63
2003	32.11
2004	32.85
2005	34.22
2006	34.47
2007	35.5
2008	36.43
2009	35.24
2010	32.98
2011	31.45
2012	31.32
2013	30.19
2014	30.12
2015	29.89
2016	29.79
2017	29.9