Census 2021 coding methodology for religion question

FOI reference: FOI-2025-2684

You asked

I note from disclosure on the 2021 census under religion that the only reference to the term "orthodox" comes within:

"Some people may have chosen to describe a denomination of one of the tick-box responses (for example, Catholic as a denomination of Christian or Orthodox as a denomination of Jewish) through the "Any other religion" write-in response option".

Given the likelihood that there are more Orthodox Christians in the UK than Orthodox Jews (Orthodox Christianity comprising two major denominations i.e., the Eastern Orthodox Church (churches such as the Moscow Patriarchate, the Romanian Patriarchate, the Ecumenical Patriarchate etc) and the Oriental Orthodox Church (churches such as the Coptic Church, the Ethiopian Tewahedo Church, the Armenian Apostolic Church etc) - and Orthodox Judaism comprising no more than half of world Jewry) I would be grateful for your methodology in determining that "Orthodox" refers to Jews and not to Christians; and for identifying which is which out of a participant's completed census (i.e., where they have ticked, "Any other religion").

We said

Thank you for your freedom of information request. 

"Write-in" answer boxes and "search-as-you-type" functions meant everyone could answer census questions as they wished. For all the identity questions, people could select one of the tick-box options or write in their answer. 

For the religion question, the hundreds of thousands of write-in answers were grouped into 144 different categories. 

Most of the write-in answers that make up these categories are denominations or branches of the "tick-box" religions. These categories are grouped together along with the tick-box answers. So, if you wrote "C of E" into the "Any other religion" write-in box, this would be grouped into the "Church of England" category, and then into "Christian" in our published dataset. 

We record every write-in answer, but we do not routinely publish counts of them because write-in counts are not population estimates. 

Write-in counts do not represent the number or proportion of the population as a whole who identify with that religion or ethnic group. For instance, the write-in count for the number of Catholics may not represent the overall population estimate of Catholics because some Catholics may have described themselves as "Christian". Or, people who identify with the same national identity may, for example, respond with different names, spellings, or variations of that national identity.

Even with similar responses aggregated, these counts are not how we present the final census data. 

The numbers we publish in our datasets are population estimates. To produce population estimates, individual answers to census questions are collated, categorised and a number of additional statistical processes are applied to adjust for people who did not respond to the census and to ensure confidentiality. 

If a respondent ticked "Any other religion" and wrote "Orthodox", this would have been coded to orthodox church and thus aggregated to Christian. But if someone wrote "orthodox Jewish", "orthodox Judaism" and "orthodox, Jewish", these were all coded to Jewish. The associated download for coding hierarchy from Census 2021 provides full details. 

If you have any further queries on this topic, please contact census.customer.services@ons.gov.uk for assistance.