De-identification

When you will meet it

It is the mandatory step whenever you test an agent on real data, build a knowledge base, or hand cases to an external model. The cost of skipping it is covered under PII; the more dangerous move is doing it and then relaxing — de-identification is frequently reversible, especially against an attacker holding side information.

Minimal example

原始：「王小明，男，34 歲，某市某科技公司工程師，
       3 月 12 日因車禍送醫，診斷為左腿骨折」

去識別化：「一名 30-39 歲男性，居住於某地區，
       於某季因車禍就醫，診斷為下肢骨折」

攻擊者若握有旁側資訊（例如知道 3 月 12 日某區發生過
一起工程師車禍），第二條照樣能對回第一條。

Watch the generalisation: specific fields become ranges (34 → 30-39, date → season), keeping most of the analytical value — but every rare combination that survives adds reversibility. De-identification is a trade between utility and exposure, not a switch.

What people get wrong

• Treating de-identification as an irreversible done state. With side information — another dataset, news coverage, insider knowledge — an attacker can re-identify individuals. Re-identification (linkage) attacks are well documented in both research and practice.
• Equating anonymisation with de-identification. Strict anonymisation demands irreversibility and is far harder; most teams actually perform pseudonymisation or generalisation, then drop all further protection under the banner of "anonymised".