Year	%
1977	27.4
1983	26.4
1987	27.5
1993	26.6
2004	28.9
2009	29.6
2011	29.9
2015	29.7
2016	29.7
2017	30.5
2018	28.4
2019	26.3
2020	26.3
2021	25.6