Year	%
1963	16.7
1964	16.7
1965	16.7
1966	17.5
1967	16.2
1968	15.7
1969	15.7
1970	16.2
1971	16.2
1972	17
1973	16.2
1974	15.7
1975	16.5
1976	16.2
1977	15.7
1978	15.7
1979	15.5
1980	16
1981	16.5
1982	17.7
1983	18.7
1984	18.2
1985	18.2
1986	18.2
1987	18.2
1988	18.2
1989	18
1990	18.2
1991	18.7
1992	18.7
1993	19
1994	18.2
1995	17.7
1996	18
1997	17.7
1998	18
1999	17.7
2000	17.5
2001	17.7
2002	18.2
2003	18
2004	17.7
2005	17.7
2006	18
2007	18.2
2008	18.2
2009	17.5
2010	18
2011	18
2012	18.2
2013	18
2014	17
2015	17.2
2016	18
2017	18.2
2018	18.2
2019	18.5
2020	17
2021	15.5