Year	Percent
1983	41.18
1984	40.71
1985	41
1986	40.31
1987	40.11
1988	40.53
1989	40.16
1990	40.06
1991	40.3
1992	38.99
1993	37.84
1994	36.9
1995	36
1996	35.27
1997	34.67
1998	34.39
1999	33.83
2000	33.53
2001	32.78
2002	32.37
2003	31.38
2004	31.25
2005	29.84
2006	29.65
2007	29.9
2008	29.34
2009	28.8
2010	28.31
2011	28.24
2012	28.19
2013	27.78
2014	28.05
2015	27.69
2016	27.38
2017	27.4