Year	%
1977	34
1978	33.3
1979	33.3
1980	32.9
1981	31.6
1982	29.2
1983	29.8
1984	31.3
1986	31.2
1987	34.2
1989	32.1
1991	31.1
1993	35.1
1995	35.2
1998	36.7
2000	35.3
2002	34.7
2003	34.9
2004	34.3
2005	33.8
2006	33.7
2007	32.9
2008	33.8
2009	33.8
2010	34.7
2011	35.1
2012	35.2
2013	34.9
2014	34.7
2015	35.4
2016	35.2
2017	35.9
2018	35.2
2019	34.6
2020	35.2
2021	34.8