Year	Percent
1981	13.5
1988	13.5
1990	12.9
1992	12.3
1994	13.5
1996	13.7
1998	14
1999	13.6
2000	13.6
2002	14
2004	14
2006	14.3
2007	14.7
2008	14.6
2009	14.8
2010	14.8
2011	15.1
2012	14.8
2013	15.1
2014	15.3
2015	15.5