Year	%
1970	37.1
1975	35.9
1979	34
1984	33.4
1990	32.1
1996	32
1997	31.9
1998	31.7
1999	32.5
2000	32.6
2001	32.5
2002	31.8
2003	31.4
2004	30.6
2005	29.8
2006	29.7
2007	32.4
2008	33
2009	32.7
2010	33.7
2011	33.3
2012	33.1
2013	32.5
2014	32.3
2015	32.7
2016	31.9
2017	31.6
2018	32.4
2019	31.2
2020	30.7
2021	31.5