Year	%
1963	27.3
1964	27.8
1965	27.2
1966	27.2
1967	26.8
1968	26.5
1969	26.3
1970	26.6
1971	27
1972	26.5
1973	26.2
1974	25.7
1975	25.8
1976	25.7
1977	26
1978	25.2
1979	25.3
1980	25.2
1981	25.7
1982	26.3
1983	26.6
1984	26.6
1985	27
1986	26.8
1987	26.5
1988	27.1
1989	27.7
1990	27.6
1991	27.5
1992	27.7
1993	29.7
1994	29.6
1995	29.7
1996	30
1997	30.4
1998	29.7
1999	29.9
2000	30.3
2001	30.6
2002	30.1
2003	30.2
2004	30.1
2005	30.8
2006	31.2
2007	30.4
2008	30.3
2009	30.4
2010	29.3
2011	30.2
2012	30.2
2013	30
2014	30.7
2015	30.5
2016	30.4
2017	30.3
2018	30.7
2019	30.8
2020	29.5
2021	30.1