Year	%
1971	23.1
1973	23.2
1975	23.3
1977	23.5
1979	23.5
1981	23.4
1982	23.1
1984	23.1
1985	23
1986	23.2
1987	23.1
1988	23.2
1989	23.3
1990	23.3
1991	23.3
1992	23.2
1993	23.3
1994	23.2
1995	23.2
1996	22.9
1997	22.7
1998	22.7
1999	22.9
2000	22.7
2001	22.8
2002	22.9
2003	22.8
2004	22.9
2005	22.8
2006	22.7
2007	22.6
2008	22.7
2009	22.8
2010	22.7
2011	22.8
2012	23.1
2013	23.3
2014	22.9
2015	23
2016	22.9
2017	22.9
2018	23
2019	23