Year	Percent
1997	18.52
1998	19.54
1999	18.31
2000	20.13
2001	19.37
2002	17.55
2003	20.33
2004	18.45
2005	18.03
2006	16.94
2007	17.29
2008	17.26
2009	17.6
2010	17.83
2011	17.04
2012	16.65
2013	16.05
2014	15.44
2015	14.32
2016	13.79
2017	13.44