Year	%
1971	37.3
1973	34.9
1975	34.1
1977	33.7
1979	33.5
1981	33
1982	32.9
1984	33.5
1985	33.2
1986	31.7
1987	31.7
1988	31.2
1989	31
1990	31.4
1991	32
1992	31.6
1993	31.7
1994	31.3
1995	31.5
1996	32.8
1997	33.1
1998	33.2
1999	33.1
2000	33.4
2001	33.6
2002	33.6
2003	33.8
2004	33.8
2005	33.6
2006	34.1
2007	33.8
2008	33.9
2009	34
2010	33.6
2011	32.7
2012	33.5
2013	33.8
2014	33.2
2015	33.7
2016	32.7
2017	33.3
2018	32.5
2019	31.7