Year	Percent
1991	25.95
1992	24.67
1993	25.1
1994	25.85
1995	24.93
1996	23.48
1997	24.63
1998	26.62
1999	23.13
2000	22.09
2001	22.86
2002	22.51
2003	21.63
2004	23.34
2005	21.62
2006	21.52
2007	20.84
2008	21.97
2009	19.42
2010	18.4
2011	18.51
2012	18.16
2013	18.57
2014	18.39
2015	17.78
2016	17.42
2017	17.44