Year	%
1981	45.2
1988	43.8
1990	45.3
1992	47.9
1994	43.5
1996	42.9
1998	41.5
1999	43.1
2000	42.8
2002	41.9
2004	42.5
2006	41.8
2007	39.8
2008	40.3
2009	39.6
2010	39.4
2011	37.5
2012	39.3
2013	37.8
2014	37
2015	36
2016	36.9
2017	36.4
2018	36.4
2019	34.9
2020	35
2021	34.9