Year	%
1990	31.4
1991	30.6
1992	29.8
1993	29.2
1994	28.6
1995	28.1
1996	27.6
1997	27.1
1998	26.7
1999	26.3
2000	25.9
2001	25.6
2002	25.3
2003	25.1
2004	24.9
2005	24.7
2006	24.4
2007	24.2
2008	24
2009	23.8
2010	23.7
2011	23.7
2012	23.8
2013	24
2014	24.2
2015	24.5
2016	24.9