Year	%
1968	26.8
1969	27.9
1970	28
1971	29.3
1972	28.8
1973	28.8
1974	29
1975	27.9
1976	27.6
1977	27.1
1978	27.2
1979	27.4
1980	28.5
1981	29.7
1982	29.9
1983	29.8
1984	29.2
1985	30.4
1986	31.9
1987	33.9
1988	34.3
1989	33.4
1990	35.7
1991	35
1992	35
1993	35.1
1994	36
1995	35.5
1996	35.3
1997	35.7
1998	36.6
1999	36.8
2000	38.8
2001	37.1
2002	35.1
2003	34.9
2004	34.8
2005	35.5
2006	35.9
2007	34.4
2008	35.4
2009	35.1
2010	33.7
2011	33.2
2012	33.1
2013	32.7
2014	33.1
2015	33.3
2016	33.1
2017	32.6
2018	33.7
2019	32.8
2020	32.6
2021	32.4