Year	%
1981	35.5
1988	35.3
1990	36.4
1992	38.6
1994	34.9
1996	34.3
1998	32.3
1999	34.1
2000	33.7
2002	33.2
2004	33.8
2006	32.5
2007	30.8
2008	31.1
2009	30.6
2010	30.7
2011	29.9
2012	30.4
2013	29.2
2014	28.7
2015	28.4
2016	28.9
2017	28.3
2018	28.1
2019	27.2
2020	27
2021	27.1