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Anonimatum

Comparison · GDPR

Anonymization or pseudonymization: the difference and when to use each

The two terms are often used interchangeably, but the GDPR treats them very differently. Pseudonymization, under Article 4(5), means processing data so it can no longer be attributed to a person without additional information kept separately. Anonymization goes further: the person can no longer be identified by any means reasonably likely to be used.

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The practical consequence is set out in Recital 26: pseudonymized data is still personal data and the GDPR still applies to it; anonymous data falls outside its scope. Pseudonymization is a valuable security and minimisation measure, but it does not make a document fit for publication. Anonymization can, at the price of never being able to go back.

In Anonimatum you can apply both to the same document with the same detection. Anonymization removes the data from the file, leaving no text hidden under a black box. Pseudonymization replaces it with a consistent substitute and produces an equivalence map that allows reversal by whoever holds it, and no one else.

Choosing the technique in three questions

  1. 1

    Will anyone need to know who is who again?

    If yes, because the document returns to a case file or the study requires re-identification, you need pseudonymization and a securely held map.

  2. 2

    Will the document reach an open audience?

    Website publication, case-law dissemination or open training material: anonymize, because any route back to a person is a risk.

  3. 3

    Does the recipient need to tell people apart?

    If redacting everything leaves an unreadable text, consistent pseudonyms keep it readable; if you also need irreversibility, do not keep the map and review the context.

Fictitious document

The same sentence, each technique

Fictitious excerpt from an internal report. First anonymized; in the last two lines, the same text pseudonymized with consistent names and dates.

Before · Internal report (fictitious document)

Anonymized:

Carmen Ibarra Solís reported on 4 March that Diego Mena had not submitted the report.

On 11 March, Diego Mena cited a sick leave he had not notified.

Pseudonymized:

Carmen Ibarra Solís reported on 4 March that Diego Mena had not submitted the report.

On 11 March, Diego Mena cited a sick leave he had not notified.

After · Internal report (fictitious document)

Anonymized:

(redacted) reported on (redacted) that (redacted) had not submitted the report.

On (redacted), (redacted) cited a sick leave he had not notified.

Pseudonymized:

Clara Iglesias Sanz reported on 19 April that Daniel Molina had not submitted the report.

On 26 April, Daniel Molina cited a sick leave he had not notified.

The pseudonymized version still shows it is the same person in both sentences and that a week passed between events; the anonymized version loses that, but there is no way back.

How Anonimatum does each one

True redaction, not a box on top

Redacted text is removed from the file and metadata is deleted. Copy-pasting or removing a black layer recovers nothing, because nothing is left underneath.

Consistent pseudonymization

The same original always yields the same substitute throughout the document. Options include realistic substitution, tokenization (NAME_1, EMAIL_001) and partial masking.

A map you hold

Each pseudonymization produces an equivalence map you can review and export as JSON or encrypted. Reversal is only possible with that map.

Sector profiles

«Justice», «Healthcare» and «Research» define how names, dates, addresses and amounts are replaced so the document remains useful in each context.

Anonymization and pseudonymization, point by point

  • What happens to the data

    Anonymization
    Removed or blacked out irreversibly.
    Pseudonymization
    Replaced with a consistent fictitious value or a token.

  • Reversible?

    Anonymization
    No. Nothing is left to recover the original.
    Pseudonymization
    Yes, with the additional information (the map) kept separately.

  • Still personal data?

    Anonymization
    No, if no one is identifiable by reasonable means (Recital 26).
    Pseudonymization
    Yes. The GDPR still applies (Art. 4(5) and Recital 26).

  • Readability

    Anonymization
    Lower: gaps make it harder to follow who did what.
    Pseudonymization
    High: consistent names and dates keep the narrative.

  • Typical uses

    Anonymization
    Publication, case-law dissemination, transparency, open training.
    Pseudonymization
    Longitudinal research, third-party review, test environments.

  • Main risk

    Anonymization
    Re-identification through context if only names are hidden.
    Pseudonymization
    Loss or leak of the equivalence map.

  • In Anonimatum

    Anonymization
    True redaction with text and metadata removed.
    Pseudonymization
    Substitution, tokenization or masking, with a JSON or encrypted map.

FAQ: anonymization and pseudonymization

Is pseudonymization the same as pseudo-anonymization?

Yes, they refer to the same concept. The GDPR uses «pseudonymisation»; «pseudo-anonymization» and «pseudonymization» are common variants.

If I delete the map, does pseudonymized data become anonymous?

Not necessarily. Without the map you cannot reverse the substitution, but if the context still points to someone, such as a unique role or a well-known event, it is not anonymous. Those clues need reviewing too.

Why isn't replacing names enough?

Because exact dates, addresses, licence plates or specific amounts can identify someone as well as a name. That is why Anonimatum's profiles also shift dates and replace addresses and identifiers.

Which technique suits sharing data with researchers?

It depends on the project, its legal basis and, in health, the ethics committee. If the same subjects must be followed across documents, pseudonymization is usual; for published results, anonymization.

Can I apply both to the same document?

Yes. You can pseudonymize the parties to keep the text readable and irreversibly redact details that add nothing, such as bank accounts or phone numbers, in the same run.

Keep reading

Not sure which technique you need?

Tell us what documents you share and with whom, and we will show you both options applied to a similar example.

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