Year	%
1997	43.8
1998	45.9
1999	49.6
2000	43.8
2001	45.2
2002	43.1
2003	42.9
2004	42.9
2005	43.8
2006	42.7
2007	42
2008	42.4
2009	40.6
2010	40.3
2011	39.3
2012	38.4
2013	37.8
2014	36.7
2015	36.9
2016	36.6
2017	36.1
2018	36
2019	36.2
2021	36