Year	%
1968	13.7
1969	13.6
1970	13.4
1971	13.2
1972	13.4
1973	13.2
1974	13.4
1975	13.6
1976	13.5
1977	13.7
1978	13.7
1979	13.6
1980	13.5
1981	13.2
1982	13.1
1983	13.1
1984	13.3
1985	12.9
1986	12.6
1987	12.1
1988	12
1989	12.2
1990	11.6
1991	11.8
1992	11.8
1993	11.8
1994	11.6
1995	11.6
1996	11.7
1997	11.7
1998	11.5
1999	11.5
2000	11.1
2001	11.4
2002	11.9
2003	11.9
2004	11.9
2005	11.8
2006	11.8
2007	12
2008	11.8
2009	11.9
2010	12.2
2011	12.3
2012	12.3
2013	12.3
2014	12.3
2015	12.2
2016	12.3
2017	12.5
2018	12.2
2019	12.4
2020	12.4
2021	12.5