Year	%
1990	39.6
1991	38.8
1992	38.1
1993	37.4
1994	36.7
1995	36
1996	35.4
1997	34.8
1998	34.2
1999	33.7
2000	33.3
2001	33
2002	32.8
2003	32.6
2004	32.5
2005	32.4
2006	32.3
2007	32.2
2008	32.2
2009	32.2
2010	32.3
2011	32.5
2012	32.8
2013	33.1
2014	33.4
2015	33.8
2016	34.4