Year	%
1971	27
1973	25.5
1975	25
1977	24.5
1979	24.4
1981	24.3
1982	24.5
1984	25
1985	24.9
1986	23.7
1987	23.8
1988	23.5
1989	23.2
1990	23.5
1991	23.7
1992	23.6
1993	23.6
1994	23.4
1995	23.6
1996	24.9
1997	25.4
1998	25.5
1999	25.1
2000	25.6
2001	25.6
2002	25.5
2003	25.8
2004	25.6
2005	25.6
2006	26.1
2007	26
2008	25.9
2009	25.8
2010	25.7
2011	25
2012	25.2
2013	25.1
2014	25.2
2015	25.5
2016	25
2017	25.3
2018	24.8
2019	24.4