Year	%
1968	17.3
1969	17.2
1970	17.1
1971	16.8
1972	17.1
1973	16.9
1974	17
1975	17.3
1976	17.3
1977	17.4
1978	17.5
1979	17.3
1980	17.1
1981	17.1
1982	17
1983	17
1984	17.2
1985	16.9
1986	16.7
1987	16.1
1988	16
1989	16.4
1990	15.9
1991	16.1
1992	16.2
1993	16.2
1994	16
1995	16.1
1996	16.2
1997	16.1
1998	15.8
1999	15.8
2000	15.2
2001	15.6
2002	16.2
2003	16.3
2004	16.2
2005	16.2
2006	16
2007	16.5
2008	16.2
2009	16.2
2010	16.5
2011	16.6
2012	16.6
2013	16.7
2014	16.7
2015	16.7
2016	16.9
2017	17
2018	16.7
2019	17.1
2020	16.9
2021	17