Re-identification risk (motivated intruder test)

Re-identification risk is the possibility that a person can be identified from data intended to obscure their identity, including by combining it with other information. The assessment depends on the data, recipient and release context. Removing direct identifiers is therefore only one part of evaluating whether information is anonymous.

Key Takeaways

• Assess combinations of characteristics and available external information, not just names and identification numbers.
• A motivated intruder test explores realistic identification attempts; it is not a guarantee against every future attack.
• The EDPB’s July 2026 anonymisation guidelines remain a consultation version on the source checked for this entry.

How Is the Risk Assessed?

The ICO describes a contextual assessment of identification risk, including a motivated intruder approach. This asks what a reasonably competent person might achieve using available means, rather than imagining unlimited resources. ONS guidance treats intruder testing as a practical technique that complements other disclosure-control assessment. The EDPB’s Guidelines 02/2026 discuss anonymisation in a consultation text, not a final binding rule. Document which framework and version inform the assessment, especially when working across EU and UK requirements.

Primary guidance: ICO anonymisation effectiveness; ONS intruder testing guidance; EDPB anonymisation consultation.

Why It Matters for Data Protection

For a hypothetical public rare-disease dataset, we recommend examining whether age, treatment dates and a specialist centre could be combined with publicly available patient accounts. Record the assumed attacker, accessible information, attempted matches and limitations of the test. Choose transformations or access restrictions that address the observed risks while preserving the research purpose. Reassess when the dataset, release audience or available information changes. A failed identification attempt is useful evidence, but should not be treated as proof that identification is impossible in every context.

Frequently Asked Questions

Does removing names eliminate the risk?

No. Indirect identifiers and combinations may still make identification possible.

Is a motivated intruder test sufficient alone?

Not necessarily. Combine practical testing with the wider risk assessment.

Are the 2026 EDPB guidelines final?

The source checked on 27 October 2026 identifies them as a public consultation version.

Related reading: anonymization; pseudonymisation; EDPB anonymisation guidance and clinical trials.

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Updated: 27 October 2026