Year	%
1980	15.4
1986	14.8
1987	14.2
1991	13.3
1992	13.9
1993	14.3
1994	14
1995	13.3
1996	13.1
1997	13.2
1998	12.7
1999	13.1
2000	12.8
2001	12.3
2002	12.2
2003	12.7
2004	13.6
2005	13.7
2006	14.2
2007	14.2
2008	14.5
2009	15
2010	14.9
2011	15
2012	15.3
2013	15.5
2014	15.2
2016	15.2
2017	15.4