Year	%
1975	18.1
1981	18.7
1987	18.3
1992	18.1
1995	18.4
2000	17.5
2001	17.5
2002	17.7
2003	18.3
2004	17.9
2005	18.2
2006	18
2007	17.8
2008	17.6
2009	17.8
2010	17.8
2011	17.8
2012	17.9
2013	17.7
2014	17.7
2015	17.6
2016	17.5
2017	17.6
2018	17.6
2019	17.6
2020	17.7
2021	17.5