Year	% of revenue
1990	27.058418059624334
1991	23.443555287130707
1992	25.23596244844252
2014	25.26548868714328
2015	26.230584173874007
2016	29.45916533013865
2017	28.499650679703343
2018	29.80593301335225
2019	28.740605977010897
2020	27.66986412524077