Year	%
1968	22.4
1969	22.3
1970	22.3
1971	22.3
1972	22.7
1973	22.4
1974	22.4
1975	22.5
1976	22.7
1977	22.6
1978	22.8
1979	22.7
1980	22.5
1981	22.5
1982	22.5
1983	22.5
1984	22.7
1985	22.7
1986	22.4
1987	22.1
1988	22.1
1989	22.8
1990	22.6
1991	22.6
1992	22.6
1993	22.7
1994	22.3
1995	22.4
1996	22.4
1997	22.4
1998	22.1
1999	22.1
2000	21.2
2001	21.7
2002	22
2003	22.2
2004	22.2
2005	22.1
2006	21.9
2007	22.5
2008	22.2
2009	22.2
2010	22.3
2011	22.5
2012	22.5
2013	22.8
2014	22.7
2015	22.8
2016	22.7
2017	22.9
2018	22.7
2019	22.9
2020	22.9
2021	22.9