Year	%
1998	16.9
1999	17.2
2000	17.2
2001	17.2
2002	17.2
2003	17.5
2004	17.9
2005	17.8
2006	17.7
2007	17.3
2008	17.7
2009	17.7
2010	17.5
2011	17.8
2012	17.9
2013	17.8
2014	17.7
2015	18
2016	17.9
2017	17.9