Year	%
1968	36.7
1969	37.5
1970	37.6
1971	38.6
1972	37.8
1973	38.2
1974	38.2
1975	37.3
1976	37.1
1977	36.7
1978	36.6
1979	37
1980	37.8
1981	38.6
1982	38.8
1983	38.7
1984	38
1985	39.1
1986	40.2
1987	41.9
1988	42.2
1989	41.1
1990	42.9
1991	42.3
1992	42.3
1993	42.3
1994	43.2
1995	42.8
1996	42.6
1997	42.9
1998	43.7
1999	43.8
2000	45.8
2001	44.3
2002	42.7
2003	42.3
2004	42.4
2005	42.8
2006	43.3
2007	41.8
2008	42.7
2009	42.5
2010	41.5
2011	41
2012	40.9
2013	40.5
2014	40.8
2015	40.8
2016	40.6
2017	40.2
2018	41.1
2019	40.2
2020	40.3
2021	40