GDPR Q31

What is the difference between anonymised and pseudonymised data?

Q31. What is the difference between anonymised and pseudonymised data?

Category: Anonymisation

Anonymisation removes the link to an individual permanently, while pseudonymisation replaces identifiers so data can still be re-linked if you have the key.

In simple terms

Anonymised data falls outside GDPR because individuals can no longer be identified.
Pseudonymised data is still personal data and must be protected.
Use strong techniques and regular testing to ensure data stays anonymous.

Anonymisation

To be truly anonymous, there must be no reasonable way to re-identify someone, considering all likely data sources.

Remove direct identifiers and assess indirect identifiers like location or unique behaviours.
Use aggregation, differential privacy, or synthetic data where appropriate.
Test anonymisation regularly because new datasets may increase re-identification risk.

Pseudonymisation

Pseudonymisation replaces identifiers with tokens or codes. It reduces risk but still counts as personal data under GDPR.

Store the re-identification key separately with strict access controls.
Use pseudonymisation to limit exposure during analytics or testing.
Combine with other security measures such as encryption.

Practical steps to take

Decide whether anonymisation or pseudonymisation suits your purpose.
Document the techniques used and the risk assessment behind them.
Limit access to original identifiers and keys.
Review new data sources that could undermine anonymity.
Explain the status of the data in your privacy documentation.

Real-life examples

Health research

A hospital anonymises patient datasets before sharing them with researchers, removing identifiers and aggregating rare conditions.

Product analytics

A software company pseudonymises user IDs so analysts can study behaviour without seeing names.

Helpful reminders

Anonymisation must be robust—if in doubt, treat the data as personal.
Keep audit trails showing who can re-identify pseudonymised data.
Update processes when technology or external data sources change.

Key takeaway

Choose the right technique for your goal—anonymisation removes data from GDPR, pseudonymisation reduces risk but still requires safeguards.