Year	%
1997	26.4
1999	30.3
2000	29.6
2001	27
2002	28
2004	21.9
2005	26.2
2006	25.5
2007	24.3
2008	22.9
2009	23.2
2011	21.8
2012	22.9
2013	22.7
2014	21.6
2015	21.3
2016	22.2
2017	21.8
2018	20.1
2019	17.8
2020	17.9
2021	17.4