Year	%
1997	29.4
1998	28.6
1999	27.7
2000	27.6
2001	27.9
2002	28.1
2003	30.3
2004	30.5
2005	31.4
2006	31.3
2007	32.8
2008	32.5
2009	31.1
2010	30.6
2011	31.1
2012	32.2
2013	32.4
2014	31.9
2015	29.7
2016	28.6
2017	29.2
2018	29.9
2019	30.2
2020	29