Explaining the new Government Statistical Service harmonised sex standard

A well-documented challenge when producing statistics is ensuring that data collected across different sets of statistics can be combined and compared in a coherent way. Users can be frustrated by inconsistency. And in potentially controversial and divisive as well as sensitive areas, such as collecting data on identity and protected characteristics, producers themselves welcome being able to draw upon a common approach.

This was highlighted in the UK Statistics Assembly recommendations where one of the top four priorities was to “Recognise the needs for UK-wide statistics and advocate for, and support, harmonised data where desirable”. 

Harmonisation is about coherence, not uniformity 

A harmonised standard is not a rule that every organisation must follow in exactly the same way. Instead, it provides an evidence-based template that helps producers align where appropriate and be transparent about the choices they make. One method for working towards this coherence is to start with clear, consistent questions and definitions when we collect data. This is why the Government Statistical Service (GSS) Harmonisation team is developing harmonised standards for data collection across sex, gender identity, ethnicity and disability. One of the really valuable things about harmonised standards is in the testing of how questions are understood by respondents. They need to be well understood to produce reliable statistics that are useful for analysts and comparable across organisations. 

The team does this by encouraging common definitions, question wording, response options and presentation. But they also must recognise that not every data collection has the same purpose or context – and that distinction really matters.

For users of statistics, this improves confidence. When data are collected and reported using clear concepts, people can better understand whether different sources can be compared, where comparison is limited, and what conclusions can reasonably be drawn. 

Today we’ve published a new harmonised standard for data on sex 

Sex is a long-established and often fundamental topic in many official statistics and data collections. But the way it is asked has been inconsistent. Different surveys have used different wording, different guidance and sometimes different underlying definitions. Often, as the Office for Statistics Regulation has noted, the concept has been conflated, with sex and gender used interchangeably. These differences can make it harder to compare data, combine evidence and explain what statistics do — and do not — show. 

The need to improve accuracy and consistency, along with a developing legal framework, has been the driver for the new sex data collection standard produced by the GSS. It gives statistical producers a clearer starting point for deciding whether they need to collect data on sex, which concept of sex as set out in legislation that they need to measure, and how to explain that choice to respondents and data users. 

We are clear that sex is a different concept from gender identity. The harmonisation team is developing a separate standard on gender identity and expects to publish that in summer 2027. 

Why there are two sex questions 

A key feature of the standard is that it includes two question designs. This is because there are two different target concepts that statistical producers may need to measure, depending on the purpose of their data collection. The standard has been developed to collect data in line with existing legal definitions of sex. 

The first question asks about sex at birth – specifically: ‘What was your sex at birth? By sex at birth we mean the sex that was recorded on your birth certificate’. This is aligned with the definition of biological sex for the purposes of the Equality Act 2010, following the UK Supreme Court ruling that sex in that Act refers to biological sex unmodified by a Gender Recognition Certificate. This question also aligns with approaches used to collect biological sex internationally, including in Canada, New Zealand and Australia. 

The second question asks about certificated sex: explicitly, sex at birth or as modified by a Gender Recognition Certificate, in line with the Gender Recognition Act 2004. This concept may be more appropriate where the purpose of the statistics relates to certificated sex.

The existence of two questions does not mean producers should collect more data. In fact, we warn against using both questions within the same data collection. It means they should make a clear, evidenced choice. Before collecting data on sex, producers should ask whether they have a clear reason to do so, which question meets the needs of users, and what legal, ethical and data protection considerations apply.

The standard provides high level guidance to producers on how to determine which question is most appropriate for them, but it is important that producers work this through themselves.

A standard built through extensive testing 

The recommended questions in the standard have been developed using Respondent Centred Design principles. The work starts with user needs but also focuses on how people actually understand and respond to questions. The research to develop the questions focused on how respondents understood the wording, interpreted the guidance and experienced the question in an online self-completion setting. The team tested with a range of people with different characteristics and perspectives.

Testing focused on whether respondents could understand the target concept – sex – being asked, answer quickly and easily, and provide information that would meet the needs of data users. The question guidance was also tested alongside the question text. 

The research also explored acceptability and trust. Respondents wanted to know why data on sex were being collected, why sex rather than gender was being asked, and why a specific definition was needed. This finding is central to the standard: clear explanations can reduce respondent burden, improve trust and support better data quality. 

The work so far has focused on online self-completion. Further testing will look at other modes, including paper questionnaires, and telephone and face-to-face interviewing, but experience shows that there is a high chance that something that works in an online context will translate well to other modes. The team will also look further at the needs of groups not fully represented in the accelerated testing programme, for example individuals with learning difficulties, and individuals with variations in sex characteristics. This staged approach reflects the importance of continuing to build the evidence base as user needs and collection contexts evolve

Clear questions support better statistics 

The sex harmonised standard is not only about wording. It is about helping statistical producers make careful, transparent decisions that support comparability, protect respondents and build public confidence in official statistics. 

I recognise the strength of feeling among many audiences on the topic of sex and the different ways people argue it can be defined and understood. But by grounding the standard in clear legal concepts, user needs and respondent testing, the work provides a practical route through a complex topic. The GSS has maintained a neutral position throughout, focusing on the evidence base and the needs of data collectors rather than on wider policy debate. The standard helps ensure that when data on sex are collected, they are collected for a clear reason, using language that respondents can understand and definitions that data users can interpret with confidence. 

More work to come 

The same approach and methodology used to develop the sex question is used across all the GSS harmonised standards. As noted, the GSS is continuing to develop a new standard for gender identity. We also recognise that user needs and the context for official data collection on sex will continue to evolve. 

Work is also progressing on disability and ethnicity harmonised standards. We are intending to publish the ethnicity standard next spring once we have concluded research to determine whether new response options (tick boxes) would be added to the standard. The ethnicity update published today indicates which requested response options are being taken forward from the consultation for further consideration for the standard. 

If you have any feedback or questions about harmonised standards, please email harmonisation@statistics.gov.uk.