Year	Percent
1988	25.03
1989	24.16
1990	26.83
1991	25.08
1992	24.26
1993	24.91
1994	22.8
1995	21.91
1996	21.58
1997	22.27
1998	21.42
1999	23.39
2000	19.88
2001	20.1
2002	22.5
2003	18.6
2004	17.97
2005	17.59
2006	18.6
2007	18.64
2008	18.76
2009	18.81
2010	18.49
2011	17.95
2012	17.52
2013	19.08
2014	19.95
2015	19.66
2016	18.62
2017	18.55