Mudraj has two main types: mudraj al-isnād and mudraj al-matn. The former concerns the joining or transformation of chain information, creating an inaccurate impression about transmission. The latter occurs when words not originally part of the prophetic report become embedded in its wording.
Isnād interpolation: Several teachers conveyed the report through distinct paths, but the retelling obscures the differences and makes the paths appear uniform.
Matn interpolation: A narrator adds an explanatory remark, but a later transmitter fails to separate the explanation from the hadith itself.
The decisive approach is to collect independent versions of the same report and compare exactly which portions are shared and which belong to a particular route.