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GDPR - Anonymisation & Pseudonymisation

This guidance provides a brief overview of the main differences between anonymisation and pseudonymisation, and how this will affect the processing of personal data.


Recital 26 defines anonymous information, as ‘…information which does not relate to an identified or identifiable natural person or to personal data rendered anonymous in such a manner that the data subject is not or no longer identifiable’.

The GDPR does not apply to anonymised information.

Anonymisation is the process of removing personal identifiers, both direct and indirect, that may lead to an individual being identified.

An individual may be directly identified from their name, address, postcode, telephone number, photograph or image, or some other unique personal characteristic.

An individual may be indirectly identifiable when certain information is linked together with other sources of information, including, their place of work, job title, salary, their postcode or even the fact that they have a particular diagnosis or condition.

Once data is truly anonymised and individuals are no longer identifiable, the data will not fall within the scope of the GDPR and it becomes easier to use.

While there may be incentives for some organisations to process data in anonymised form, this technique may devalue the data, so that it is no longer of useful for some purposes. Therefore, before anonymization consideration should be given to the purposes for which the data is to be used.

The ICO’s Code of Conduct on Anonymisation provides a further guidance on anonymisation techniques.

The ICO’s Code suggests applying a ‘motivated intruder’ test for ensuring the adequacy of de-identification techniques.

For researchers, further guidance on how to apply the motivated can be found on the SLMS website.


Pseudonymisation is not the same anonymisation.

Pseudonymisation is defined within the GDPR as “the processing of personal data in such a way that the data can no longer be attributed to a specific data subject without the use of additional information, as long as such additional information is kept separately and subject to technical and organizational measures to ensure non-attribution to an identified or identifiable individual” (Article 4(3b)).

Unlike anonymisatoin, pseudonymisation techniques will not exempt controllers from the ambit of GDPR altogether. It does however help UCL meet their data protection obligations, particularly the principles of ‘data minimisation’ and ‘storage limitation’ (Articles 5(1c) and 5(1)e), and processing for research purposes for which ‘appropriate safeguards’ are required.

To ascertain whether means are reasonably likely to be used to identify the natural person, account should be taken of all objective factors, such as the costs of and the amount of time required for identification, taking into consideration the available technology at the time of the processing and technological developments (Recital 26).

Recital 26 provides that “Personal data which have undergone pseudonymisation, which could be attributed to a natural person by the use of additional information should be considered to be information on an identifiable natural person.”

Both the above sections of Recital 26 mean that pseudonymised personal data can still fall within scope of the GDPR.

Further reading

The UK Anonymisation Network (UKAN)
UK Data Archive