Year	%
1991	38.2
1992	38
1993	37.5
1994	37.7
1995	37.4
1996	37.1
1997	37.2
1998	37.1
1999	37.9
2000	37.7
2001	38.8
2002	38.6
2003	38.7
2004	38.9
2005	40.1
2006	39.6
2007	39.8
2008	39.5
2009	38.9
2010	38.8
2011	39.3
2012	39.4
2013	39.8
2014	39.1
2015	39.6
2016	39.4
2017	39.6
2018	39.9
2019	39.8