Year	%
1981	25
1986	19.6
1989	21.2
1990	21
1991	22.2
1992	20.3
1993	20.1
1994	19.7
1995	20.1
1996	20.4
1997	20.1
1998	19.6
1999	20.2
2000	20.7
2001	20.9
2002	20.7
2003	21.6
2004	20.1
2005	19.6
2006	19.3
2007	18.5
2008	18.6
2009	20.2
2010	18.3
2011	20.3
2012	19.9
2013	19.6
2014	19.7
2015	18.6
2016	19.5
2017	18.7
2018	19.3
2019	19.1
2020	20.3
2021	19.6
2022	18.7