Year	%
1968	22.3
1969	23
1970	23
1971	23.8
1972	22.9
1973	23.3
1974	23.5
1975	22.6
1976	22.2
1977	22
1978	21.9
1979	22
1980	22.9
1981	23.6
1982	23.7
1983	23.6
1984	23.2
1985	23.8
1986	25.3
1987	26.7
1988	26.9
1989	25.7
1990	27.5
1991	26.7
1992	26.6
1993	26.8
1994	27.8
1995	27.3
1996	27.2
1997	27.6
1998	28.5
1999	28.7
2000	31.1
2001	29.3
2002	27.7
2003	27.3
2004	27.3
2005	27.8
2006	28.3
2007	26.5
2008	27.6
2009	27.4
2010	26.4
2011	25.9
2012	25.6
2013	25.3
2014	25.6
2015	25.7
2016	25.5
2017	24.9
2018	25.8
2019	25.1
2020	25
2021	25