Year	Percent
1992	18.96
1993	19
1994	18.76
1995	20.19
1996	20.3
1997	18.62
1998	18.24
1999	17.94
2000	16.94
2001	17.11
2002	17.25
2003	17.43
2004	18.26
2005	17.72
2006	17.72
2007	18.03
2008	18.26
2009	17.5
2010	15.92
2011	16.06
2012	15.46
2013	15.22
2014	14.94
2015	14.93
2016	15.66
2017	15.7