Year	%
1971	43.4
1973	41.5
1975	40.9
1977	40.5
1979	40.3
1981	40.1
1982	40.3
1984	40.6
1985	40.4
1986	39.2
1987	39.3
1988	38.9
1989	38.6
1990	38.8
1991	39.3
1992	39.1
1993	39.1
1994	39
1995	39.1
1996	40.2
1997	40.6
1998	40.7
1999	40.4
2000	40.7
2001	40.9
2002	40.8
2003	41
2004	40.9
2005	40.9
2006	41.3
2007	41.2
2008	41.2
2009	41.2
2010	40.9
2011	40.2
2012	40.6
2013	40.6
2014	40.4
2015	40.9
2016	40.2
2017	40.6
2018	40
2019	39.4