Year	%
1963	37.6
1964	38.1
1965	37.5
1966	37.8
1967	36.9
1968	36.3
1969	36
1970	36.6
1971	36.9
1972	36.7
1973	36.1
1974	35.5
1975	35.7
1976	35.6
1977	35.5
1978	34.8
1979	34.8
1980	34.7
1981	35.5
1982	36.7
1983	37.3
1984	37.3
1985	37.6
1986	37.6
1987	37.2
1988	37.7
1989	38.2
1990	38.3
1991	38
1992	38.4
1993	40.4
1994	40
1995	39.9
1996	40.3
1997	40.5
1998	40
1999	40
2000	40.1
2001	40.6
2002	40.4
2003	40.8
2004	40.3
2005	41
2006	41.4
2007	40.8
2008	40.8
2009	40.6
2010	40
2011	40.9
2012	40.9
2013	40.7
2014	41.5
2015	41.2
2016	41.1
2017	41.2
2018	41.4
2019	41.5
2020	39.7
2021	39.8