CIPP/US Study Guide
Chapter 3: Introduction to Technological Aspects of Privacy

Deidentification: Anonymous vs Pseudonymous and Identifiers

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When data cannot be traced to a person, privacy law no longer applies. Anonymization removes identifiability; pseudonymization masks identity with a unique identifier. EU law treats pseudonymized data as personal under the GDPR, but anonymized data falls outside it. Identifiers range from strong identifiers to quasi-identifiers.

Privacy laws apply to personal data; when data can no longer be traced to a person, they no longer apply. Anonymization removes identifiability, while pseudonymization masks identity behind a unique identifier (e.g. 'Patient 13579'). EU law treats pseudonymized information as personal data under the GDPR, in contrast to anonymized data, which falls outside the GDPR.

Types of identifiers
IdentifierExample / nature
Strong identifierSocial Security or passport number; clearly identifying
Weak identifierMust be combined with other information to identify
Quasi-identifierDate of birth - over 25,000 cells (366 days x 80+ years) make it highly identifying when combined with external knowledge
Linked vs linkable

FTC Chair Ramirez (2016): data is personally identifiable when it can be reasonably linked to a particular person, computer, or device - including persistent identifiers like device IDs, MAC addresses, static IPs, and loyalty card numbers. Linked data already ties to identity; linkable data could be tied to it.

Key terms - quick answers

What is “Anonymization”?
Putting data in a form that does not identify individuals and where identification by combination with other data is not likely (UK ICO definition).
What is “Pseudonymization”?
Distinguishing individuals in a dataset using a unique identifier that does not reveal their real-world identity (UK ICO definition).
What is “Deidentified”?
Data altered so it is no longer identifiable; it is far from simple to determine when data is truly deidentified.
What is “Strong identifier”?
Clearly identifying information such as a Social Security or passport number.

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