Anonymisation vs pseudonymisation: differences and why they matter under the GDPR

An everyday example

Imagine a patient list. If you replace each name with a code and keep the “code ↔ name” table in a safe, you have pseudonymised: whoever holds the key can re-identify. If you destroy the table and the remaining data cannot identify anyone, you would have anonymised.

Key differences

• Reversibility: pseudonymisation can be reversed with the additional information; effective anonymisation cannot.
• Legal regime: pseudonymised data remains personal data; anonymous data falls outside the GDPR.
• Usefulness: pseudonymising keeps more useful detail; anonymising usually reduces it.

Common techniques

• Codes or tokens replacing identifiers.
• Generalisation: age ranges instead of exact age.
• Aggregation: working with totals rather than individual records.
• Suppression of identifying fields.

The re-identification risk

The key question is whether, by combining the data with other data, someone could identify a person using reasonable means. Removing name and ID number is rarely enough. The more detailed and unique the data, the higher the risk.

What to do

1. Decide what you really need the data for.
2. Use the minimum level of detail.
3. If you pseudonymise, keep the additional information separate with restricted access.
4. If you say you anonymise, document how you assessed re-identification.
5. Review with your DPO or adviser before treating data as anonymous.

Frequently asked questions

What is pseudonymisation?

Separating identity from the data; it remains personal data (Art. 4(5)).

Is removing the name and ID enough?

Usually not: the re-identification risk must be assessed.

Informational content, not legal advice.