diff --git a/concepts.md b/concepts.md
index d5075e1..062e927 100644
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@@ -134,6 +134,10 @@ Equality of outcomes
 | overlapping characteristics              | all inequalities can be intersectional through individuals' overlapping disadvantaged characteristics or situations                                                                                               |
 | enabling of opportunity/outcome equality | two philosophies of seeking equality, by either providing a 'level playing field' (opportunity) or ensuring equality in the resulting situations (outcome)                                                        |
 
+Missing: 
+
+- difference between relative and absolute inequality (see @Ravallion2018inequalityglobalizat, 637)
+
 # Inequalities in the world of work
 
 ## Income Inequality