Developing sex and gender identity harmonised standards: findings from phase 1 of our research

About the GSS Harmonisation Team

The Government Statistical Service (GSS) Harmonisation team is based in the Office for National Statistics (ONS). The team:

• is responsible for creating and embedding the UK Government’s “harmonised standards” for all topics
• supports the harmonisation of data across the UK

Our work involves providing bespoke advice and harmonised standards and guidance about how data on different topic areas should be collected and presented to help support statistical producers. The Code of Practice for Statistics recommends that organisations producing official statistics should seek to design new statistics in ways that achieve UK comparability and improve consistency and coherence with related statistics but does not mandate the use of harmonised standards. Others may also choose to use these standards where they help the consistency, comparability and coherence of data and statistics.

Statistical producers can use the harmonised standards as a starting point in their data collection processes. The standards are designed to promote alignment across organisations, not to enforce identical approaches. Because of this flexibility, they can be adapted to suit specific needs or contexts, including the use of administrative data sources for research. By applying harmonised standards, statistical producers can:

• work in greater alignment with others
• increase the usefulness of their statistics
• provide confidence that their statistics are produced in robust ways, in line with the Code of Practice for Statistics Standard Six: Be Rigorous

Harmonised standards allow people to effectively and accurately compare data that has been collected across different datasets. This includes primary research data and the secondary use of administrative data for research purposes. This means we can more easily understand what those data do, and do not, tell us. It also ensures that statistics are being used to their full effect for the public good.

The priority topic areas for Harmonisation align with the 2021 report of the independent Inclusive Data Taskforce (IDTF), which recommended regular reviews of harmonised standards for relevant groups and populations, such as those with protected characteristics. Specifically, Harmonisation work aligns with the IDTF’s recommendations around ensuring people can recognise themselves and their circumstances in the UK’s data collection tools and processes.

About this report

This report summarises the first phase of discovery research and engagement activities undertaken by the GSS Harmonisation Team as part of a new phase of work to update harmonised standards and guidance for the collection of sex and gender identity data.

We conducted desk research to better understand how sex and gender identity data are currently collected, both in the UK and internationally, the terminology and definitions in use and the extent of consistency across surveys. We found that:

• there are inconsistencies in the way sex is defined across UK surveys, which poses risks to data quality and to the coherence of statistics
• gender identity is an evolving topic and data collection methodologies across the UK and internationally are inconsistent

We have engaged with cross-government data collectors and producers of published statistics, to understand their needs for new harmonised standards. We also held a series of listening events across the UK in Spring 2025 to hear from data users, statistical producers and members of the public interested in data and statistics. As sex and gender identity data collection is important internationally, we have also engaged with international colleagues from National Statistical Offices to learn from their approach to sex and gender identity data collection.

We are currently completing qualitative research with respondents to understand how individuals with a range of backgrounds and perspectives conceptualise the topics of sex and gender identity. This means learning about what they draw upon to reach their understanding of different terminology and concepts, the thought processes that take place when surveyed, and how different questionnaire response options are understood.

The work we undertake to develop harmonised standards for the topics of sex and gender identity follows Respondent Centred Design principles. This emphasises the importance of learning gained during the discovery phase, which focuses on understanding the problems that need to be solved for users and respondents. You can learn more about the discovery phase of survey development in the survey development toolkit.

Our approach to communication and engagement

We aim to ensure transparency and inclusivity in our communication and engagement. Our GSS harmonisation workplan explains how we will work towards establishing agreed harmonised standards for sex and gender identity statistical data collection.

We launched this phase of work with a series of stakeholder engagement events to hear from data users, statistical producers, and respondents on their needs for data on these topics. Between April and May 2025, we held five listening events across the UK in Edinburgh, London, Cardiff, Belfast and Manchester. The purpose of these events was for us to listen to a wide range of perspectives on the topics of data for sex and for gender identity. ​This was the first stage of our work to develop new harmonised standards on these topics.​

These events provided a valuable opportunity for us to hear directly from data users and members of the public. This allowed us to gather early insights into a range of perspectives on the topics of sex and gender identity, including diverse reflections on:

• the difficulty of working in this topic space, and the awareness of potential challenges to developing appropriate questions​
• the importance of public trust in official statistics, highlighting the need to produce clear standards​
• the potential need for the development of multiple approaches to data collection, to fully capture the nuances in this topic space

Insights from this engagement, alongside our other discovery work and engagement activities, will help inform the design and focus of our upcoming research and ensure that:​

• the questions we ask in our discovery research and engagement activities are meaningful and understandable, and explore a full range of themes
• we identify and recruit a wide range of users and respondents with different perspectives, backgrounds and life experiences​
• we identify complex and contested aspects of work for deeper exploration to fully understand needs and challenges

To ensure our work reflects the full diversity of user and respondent needs, we will:​

• test these perspectives in future research, using them as a starting point to explore whether and how they are expressed across a broader and more diverse population
• seek out perspectives that may not have been present in this engagement, ensuring we identify and address any gaps in our understanding
• develop our work taking account of what we have heard so far, testing our understanding of different stakeholder views and exploring all relevant angles, not just those already raised

Updates on our work will be published regularly throughout the development cycle. If you are interested in being kept informed of our progress, or would like to contribute to any of the phases of work described in this report, please contact Outreach.Engagement.Comms@ons.gov.uk.

Desk research

We conducted desk research as part of our initial discovery phase to gather data to: ​

• understand what relevant research and information already exists on the topics of sex and gender identity statistics and data collection
• identify any gaps in current knowledge

Our desk research encompassed a wide range of perspectives to explore the broad context of statistical data collection on these topics. ​We sought to understand:

• how sex and gender identity data are currently collected, both in the UK and internationally
• the terminology and definitions in use

This work also aimed to identify any gaps in knowledge and provide us with a further understanding of different approaches being applied within in the field of data collection and statistics.

Discussion

Sex data collection

Sex data are collected across many UK and international surveys. Sex has been a core UK census topic since 1801, with the question wording “What is your sex?” remaining largely unchanged over recent decades. Most UK surveys we looked at provide binary response categories to their sex data collection questions, with ‘male’ and ‘female’ being the typical response options.

However, the way that sex is defined, explicitly or implicitly, has seen variation over time across surveys in the UK. This includes variation in terminology (for example, referring to gender instead of sex), and variations in guidance (for example, for a trans or non-binary respondent on how to interpret the sex question). Some surveys also include variation in response options (for example, including other response options such as ‘intersex’ and ‘non-binary’). In line with UK census questions on sex, 71 questions in UK surveys included only ‘female’ and ‘male’ response options. Six questions used similar binary response options but instead used other terms such as ‘men’ and ‘women’, or ‘boys’ and ‘girls’. Thirteen questions on sex, from the sample of surveys we considered, included response options beyond the binary ‘male’ and ‘female’. These options included ‘intersex’, ‘non-binary’, ‘other’, and ‘in another way’. We also saw this in some international censuses where there was/is a single question asked on sex which included three or more responses, for example including response options ‘transgender’ or ‘other’.

This blurring of terminology and definitions has an impact on the comparability of data across collections. Where the differences between collections are not well communicated, this can lead to confusion for data users and affect the quality of analysis.

The inconsistencies in definitions, response options and accompanying guidance to a question on sex may also affect how respondents interpret and answer the same question across different survey contexts. Such variation poses a challenge for data comparability, and highlights the importance of clear, consistent, and inclusive guidance in future data collection for sex.

This risk to data quality and to the coherence of statistics is one of the main purposes for the work of the GSS Harmonisation team. Similar findings on data inconsistencies were also published in the Independent Review of Data, Statistics and Research on Sex and Gender commissioned by the Secretary of State for Science Innovation and Technology in March 2024 and published in 2025. The Office for Statistics Regulation’s guidance on collecting and reporting data and statistics about sex and gender identity in official statistics states that such data should be explained and defined for the purpose of a particular set of statistics, and terms, including gender, should not be used interchangeably or as a substitute for each other.

Given the variations we have observed in the way that sex data are collected and the possible implications for data comparability, the GSS Harmonisation team are accelerating work on question development and testing for sex data and expect to publish harmonised guidance and standards in Summer 2026. We will iteratively develop and test candidate questions aligned to priority statistical user needs for data on sex and conduct cognitive and usability testing with participants of diverse backgrounds and characteristics to ensure that question wording and associated respondent guidance are easily understood. This will enable us to rapidly produce clear and evidence-based recommendations for question design and respondent guidance, while also supporting producers and users of existing questions to understand the consequences of change for their own data series comparisons.

Variations of Sex Characteristic (VSC) data collection

Our desk research activities highlighted a notable gap in government and academic research focused on the experiences, needs, and appropriate data collection methods for populations with variations of sex characteristics (VSC), also known as intersex. There is currently no consensus on where or how to collect data on populations with VSC. The circumstances of VSC individuals are typically not addressed in the survey design or guidance associated with sex questions employed within official data collection. This means VSC individuals are less visible in official statistics, and this may affect their representation for services and policies. Other individuals, however, may not want to disclose their VSC status. There should be a legitimate need and clear purpose when considering whether to include collection of data which reveals sensitive personal characteristics, including data concerning health and data concerning individual sex characteristics and traits. The purpose of data collection and the ways in which these data will be used should be clearly communicated to respondents.

Our research is clear that individuals with variations of sex characteristic (VSC), or sometimes referred to as intersex traits, should not be classified as transgender​, although some individuals may identify as both intersex and as transgender. This is reflected in the revised coding of ‘intersex’ responses for Scotland’s Census 2022. Responses were coded from ‘Other trans status or history’ to ‘No – not trans and does not have a trans history’ after feedback​.

The VSC population in the UK is small and any statistical collection seeking to gather insight into individuals with VSC should consider the effect of response errors in the non-VSC population. That is, where a rate of false positive responses (people incorrectly identifying themselves as members of this group) may overwhelm the true positives. The consequence of such false positive response errors could be to further restrict visibility of the true population, leading to poor quality decisions. Given this risk of negative outcomes, and the range of variations and life experiences, the GSS Harmonisation team will prioritise further discovery work with this population. We expect this to take the form of in-depth qualitative research with people with VSC, exploring terminology, data risks and user needs.

Gender identity data collection

Measuring gender identity is an evolving topic, and its inclusion in large-scale data collection is still developing. This lack of agreed best practice is discussed in interim harmonisation guidance for collecting gender identity data.

In the UK, when gender identity data are collected, methods of data collection are often inconsistent. There is no clear consistency across data collection methods on terminology or definitions. Data collectors may be using different question wording to measure the same concepts, which may lead to increased confusion among respondents and reduced data quality.

Inconsistent question wording was evident in the questions we reviewed in the UK. This inconsistency affects both the way gender identity is measured and data collection. For example, the term ‘gender’ appears in some questions, whilst others will ask for ‘gender identity’. These issues highlight the importance of careful question development, informed by respondent centred design research, to ensure clarity, inclusivity, and data quality.

In the material we reviewed, a range of concepts are referred to and defined. Some of the ways these concepts were defined include:

• gender identity – this can be defined as a person’s innate sense of their own gender, which may or may not correspond to their sex registered at birth
• gender reassignment (as used in the Equality Act 2010) – this means proposing to undergo, undergoing, or having undergone a process to reassign your sex
• non-binary – this is an umbrella term used to describe gender identities where someone does not identify exclusively as a man or a woman
• trans or transgender – this is an umbrella term used to refer to people whose gender is different from their sex at birth
• trans status – which refers to whether or not an individual considers themselves to be trans (in Scotland’s Census 2022 question on trans status and history, guidance was included for the write-in response that respondents could record non-binary as a trans identity)

The term ‘gender’ was particularly inconsistent, and was referred to in several ways within the material we reviewed. It was used:

• as a synonym for ‘sex’, within questions designed to collect binary male/female or man/woman responses
• as a way of collecting data beyond binary male/female or man/woman responses, for example including response options for male, female, transgender, nonbinary, and write-in options
• as a social construction relating to a set of norms, roles and relationships that is founded in social mores, laws, processes and policies based on labels of masculinity and femininity, which are time and culture specific

For the purposes of this review, we grouped uses of the term ‘gender’ which allowed for more than binary response options, with questions which used the term ‘gender identity’. We refer to them as gender identity questions in this report. Further work by the GSS Harmonisation team will address uncertainties arising from the different definitions of gender which may be adopted by respondents and the impact on question design.

The first collections of gender identity, or trans status or history data at a UK country population level took place through voluntary questions in the 2021 Census for England and Wales, and Scotland’s Census 2022. The Northern Ireland Research and Statistics Agency (NISRA) did not ask a question on gender identity in Census 2021 in Northern Ireland, referencing a limited user need for the topic.

Internationally, we identified multiple countries that have included concepts related to gender identity in their questionnaires. The countries we identified were Argentina, Canada, Ecuador, Malta, Nepal, New Zealand and Pakistan.

Approaches to gender identity data collection vary across the censuses we reviewed, with some countries employing a two-step format. For example, some censuses first ask whether a respondent’s gender is the same as their sex registered at birth and then allow for further self-identification through a write-in response or a list of closed responses. Others offer a broad range of gender identity categories, including ‘non-binary,’ ‘trans,’ ‘another gender,’ and culturally specific terms.

In censuses with separate questions on gender identity and sex, the question on gender identity was often asked directly after sex (as seen in Argentina in 2022, Canada in 2021, Malta in 2021, and Scotland in 2022). In contrast, the New Zealand 2023 census asked a question about the respondent’s gender directly before a question on their sex in their individual form, and it replaced the sex question in their dwelling form.

Our discovery work across UK and international sources indicates a lack of consensus on the priority concepts and associated terminology for data collection on gender identity. At the same time, the National Statistician’s Inclusive Data Taskforce in 2021 identified a significant data gap for transgender, non-binary and gender-diverse people whose lives and experiences were often missing from official data. The Taskforce underlined the important need for data collectors to provide meaningful categories that respondents can recognise and use to describe themselves and their circumstances. As has been shown in the additional guidance and quality information for users of gender identity estimates from Census 2021 in England and Wales, the way that such categories are collected must also take into account the way that different terminology will be interpreted by respondents outside the target community.

The GSS Harmonisation team are currently reviewing the findings of additional qualitative research undertaken to explore how diverse respondent groups conceptualise topics of sex, gender and gender identity – known as mental models – to add to our evidence base. Based on learning from this research we intend to prioritise further rounds of discovery research before moving into question design. We are also undertaking further work to clearly identify user needs for statistics on these topics, which are more recent areas of exploration in government data. This will include the needs of data producers, data analysts, and respondents.

Trans history

Our research highlighted that there is a subset of individuals within the transgender population who, after receiving a Gender Recognition Certificate (GRC), no longer identify with the term “transgender” and instead identify solely with their acquired gender. These individuals may interpret gender identity questions differently from those who currently identify as transgender but have not undergone legal transition. Similarly, some academic research has highlighted that the term “transgender” is often conceptualised as individuals who have not yet undergone any medical treatment, but whose gender identity and biological sex is misaligned.

Limited research exists on this group, however this distinction is an important one for further exploration. It is important to ensure that data collection methods are reflective of the diversity of lived experiences within the trans community.

User needs for future harmonised standards on sex and gender identity

Our work with data collectors and statistical producers across government has identified a range of user needs for future harmonised standards on sex and gender identity. Our work with statistical users and respondents is also highlighting different issues with current sex and gender identity data collection. We are currently reviewing the findings of our latest qualitative work with respondents, and roundtable research with data users, and expect to publish further information on user needs at a later date.

Preliminary user needs we have collected include support and guidance needs to assist the collection of high quality data on sex and gender identity, for example, on:

• terminology and concepts
• question placement and ordering, both within a survey and between questions
• the applicability of proxy responses for sex and gender identity data collection
• data handling and presentation after collection

Users have raised a range of needs for data collection topics. Priority user needs for data collection on sex particularly emphasised a heightened requirement for standardised questions that would enable compliance with legislation and official frameworks. Official guidance and legislation for data collectors to consider when determining what their data needs on this topic are, include the:

• Equality Act 2010 (which legally protects people from discrimination in the workplace and in wider society based on sex, among other protected characteristics)
• Gender Recognition Act 2004 (which enables people to change their legally recognised sex by obtaining a Gender Recognition Certificate, or GRC)

In April 2025, the UK Supreme Court ruled that sex, for the purposes of the Equality Act, refers to biological sex unmodified by a GRC. This means that sex-based protections under the Equality Act 2010 must be based on biological sex, that is, the sex of a person at birth. To support statistical producers currently considering whether and how to make changes to data collection on sex, we are prioritising testing for a harmonised standard which supports data collection for:

1. Biological sex (sex at birth)
   and
2. Certificated sex (sex at birth or as modified by GRC)

User needs for data collection on gender identity emphasised the need for questions and respondent guidance that are inclusive of diverse and minority gender identities, and that encourage responses from respondents of many different backgrounds and experiences.

The requirement for statistics and data on gender identity or trans status is more recent within government data. We will be undertaking further work to clearly articulate the user needs for statistics on these topics. This will include the needs of data producers, data analysts, and respondents, to ensure that data collection designs are clear, accessible, and fit for purpose.

What happens next

This report summarises phase 1 of our discovery phase. We have moved into the Alpha phase for our work towards a sex harmonised standard. Our work with statistical producers and data users has found that there are clear priorities across the Government Statistical Service to be able to collect data aligned with the current legal positions, as set out in the Gender Recognition Act 2004 and the Supreme Court ruling on the definition of sex for the Equality Act 2010.

We are currently focusing on further research and engagement activities to extend our understanding through a second phase of discovery to better understand user needs for, and respondents’ own interpretation of, terminology and questions on gender, trans status, and gender identity. Additionally, we will continue to explore other topics related to sex such as variations of sex characteristics (VSC, sometimes referred to as intersex, or differences of sex development). The second phase of discovery includes:

• data user roundtables for the academic and third sector
• further discovery work exploring concepts of gender, trans status, and gender identity with respondents and data users
• further qualitative research and engagement with VSC, sometimes referred to as intersex populations

We recognise that user needs and the context for official data collection continues to evolve, and therefore our guidance may need to change further after this date as circumstances change. The GSS Harmonisation Team will continue to work closely with statistics teams across the UK to understand their needs for the new standards, and incorporate this information into our user needs mapping.

Timelines

We commenced respondent testing for sex data collection questions in Spring 2026. This work has been accelerated, reflecting the urgent user need for statistical guidance shared with us by official statistics producers. Cognitive and usability testing with participants of diverse backgrounds and characteristics, ensures a rigorous evidence base for our new recommended standard, which we will publish in Summer 2026.

We expect to carry out further discovery work exploring concepts related to gender reassignment, trans status, and gender identity during 2026. We also expect to carry out further qualitative research and engagement with VSC, sometimes referred to as intersex, populations at this time. We will publish an update on this Discovery work in late 2026, and expect to commence respondent testing for gender identity questions at this time.

If you are interested in being kept informed of our progress, or would like to contribute to any of the phases of work described in this report, please contact Outreach.Engagement.Community@ons.gov.uk.

Contact us

If you would like to be involved with this work, please contact Outreach.Engagement.Comms@ons.gov.uk.