About this guidance

At a glance

• Anonymisation is a privacy-friendly way to harness the potential of data.
• Anonymising personal data is possible in many circumstances. Whether you can effectively anonymise personal data depends on the techniques you use. You should reduce the risks of identifying people to a sufficiently remote level that the information is effectively anonymised.
• This guidance will help all organisations that want to anonymise personal data, for whatever purpose.
• It will help you identify the issues you should consider to use anonymisation techniques effectively. 

In detail

• Why have you produced this guidance? 
• What is this guidance about?
• Who is this guidance for?
• How is this guidance structured?

Why have you produced this guidance?

We understand the benefits that sharing personal data can bring to organisations, people and society as a whole. But there are risks too. Effective anonymisation techniques provide a privacy-friendly alternative to sharing personal data.  

This guidance sits alongside our data sharing code of practice, which gives practical guidance on how to share personal data in line with data protection law. Anonymisation offers an alternative way to use or share data by making sure that people are not identifiable. 

What is this guidance about? 

This guidance will help you develop your understanding of anonymisation techniques, their strengths and weaknesses, and the suitability of their use in particular situations. It:  

• explains what we mean by anonymisation and pseudonymisation;
• details how this affects your data protection obligations and responsibilities;
• discusses what you should consider when anonymising personal data;
• provides good practice advice anonymising personal data; and
• discusses technical and organisational measures to mitigate the risks to people when you do so.

This guidance deals with the role that anonymisation plays in the three regimes of data protection law:  

• general processing under Part 2 of the Data Protection Act 2018 (DPA 2018) and the UK General Data Protection Regulation (UK GDPR);
• law enforcement processing under Part 3 DPA 2018; and
• intelligence services processing under Part 4 DPA 2018.

Where relevant, the guidance highlights and explains any differences between the regimes.

However, it is not intended as an exhaustive guide to data protection compliance and it is not prescriptive. It gives you the flexibility to implement anonymisation techniques in your own way, taking a proportionate and risk-based approach.

This guidance does not generally consider the impacts of anonymisation on areas of ICO work outside data protection. However, if you are also a public authority or a public body, you should follow this guidance when considering disclosing or allowing re-use of anonymous datasets under the Freedom of Information Act 2000 (FOIA), Environmental Information Regulations 2004 (EIR), or the Re-use of Public Sector Information Regulations 2015 (RPSI). 

This guidance is not a statutory code. It contains advice on how to interpret relevant law on anonymisation and pseudonymisation. It also contains good practice recommendations.

There is no penalty if you don’t follow these recommendations. But you must find another way to comply with the law when you produce and disclose anonymous information. You may also find alternative methods that go beyond the good practice measures we set out.

However, when we look into an issue about anonymisation, we will take this guidance into consideration.

This guidance does not describe every possible anonymisation technique in detail but it includes case studies and good practice recommendations. It applies to all mediums, including tabular data, free text, video, images, and audio (including speech).  

Who is this guidance for?

You should use this guidance if you are considering turning personal data into anonymous information. For example, this guidance is relevant if you: 

• are required by law to publish anonymous information (eg some health service bodies);
• are looking to use data in new and innovative ways (eg to improve services or design new products or collect large volumes of data to train AI models);
• need to comply with a request for information under FOIA or EIR or a request for re-use under RPSI, and it includes personal data;
• want to become more transparent and accountable to people; or
• want to provide anonymous information for research purposes, or to enable wider societal benefits.

How is this guidance structured?

This guidance is divided into sections that cover different aspects of anonymisation in data protection law.  

The first section introduces the key concepts of anonymisation and pseudonymisation, places them in the context of the UK legal framework, and explains the role they play. 

The second section covers the concept of identifiability, including approaches such as the ‘spectrum of identifiability’ and how these can apply in data sharing scenarios. This section also looks at how you can manage identification risk, and covers established concepts like the ‘reasonably likely’ and ‘motivated intruder’ tests. 

The third section looks at how pseudonymisation can help you achieve data protection compliance and which technologies can provide effective pseudonymisation.

The fourth section considers accountability and governance requirements in the context of anonymisation, including data protection by design, data protection impact assessments (DPIAs) and the use of trusted third parties.  

The fifth and final section includes case studies providing practical examples of effective anonymisation. 

In each section, we discuss what you must do to comply with data protection law, as well as what you should do as good practice.  

If you are a reader with a general interest in understanding the concepts of anonymisation and pseudonymisation: read the first section. 

If you are a technical expert and want to understand whether the technology you use results in anonymous or pseudonymous data: read the second and third sections.

If you are a decision maker looking for information about the types of techniques available for anonymisation and pseudonymisation: read the first and fourth sections.