Chapter 3: Introduction to Technological Aspects of Privacy
Deidentification: Anonymous vs Pseudonymous and Identifiers
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
Identifier
Example / nature
Strong identifier
Social Security or passport number; clearly identifying
Weak identifier
Must be combined with other information to identify
Quasi-identifier
Date 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.