Year	%
1991	29.5
1992	29
1993	28.5
1994	29.2
1995	28.7
1996	28
1997	28.3
1998	28.1
1999	29
2000	28.8
2001	29.9
2002	29.9
2003	29.9
2004	30.2
2005	31.6
2006	31
2007	31.1
2008	30.9
2009	30.5
2010	30.3
2011	30.7
2012	31.1
2013	31.5
2014	30.9
2015	31.4
2016	31.4
2017	31.9
2018	31.8
2019	31.7