Deterministic matching

Deterministic matching is the simplest form of entity resolution: if two records share a key field exactly, they're the same entity. Otherwise, they're not.

It works perfectly when every record has a clean shared identifier (national ID, MRN, tax ID) and never the wrong value in that field. It breaks the moment the identifier is missing, typo'd, formatted differently, or absent from one of the sources.

In practice, most real-world deduplication uses a hybrid: deterministic on the few clean identifier fields PLUS probabilistic on the rest. The deterministic part wins when it can; probabilistic handles the long tail.