What do residents of Hackney and Tower Hamlets have in common with those of Carlisle and Ceredigion? It sounds like a set-up for a very niche joke – but they’re actually the four local authorities in England and Wales that best fulfil the Office for National Statistics’ criteria for rehearsing its biggest, most challenging survey: the census.

Held every ten years, the census is an attempt to survey the entire population, and it’s compulsory to fill it in. The next census is still a while off in 2021, but the ONS has already begun rehearsing for it in four areas that best represent the varied challenges of running a survey of this scale (such as Welsh language concentration, ethnic minority populations, mobile connectivity and the number of second homes).

One of the major changes since the 2011 census is to make online forms the preferred method. On the previous occasion, just 16.4 per cent of respondents filled the form in online (lower than the 25 per cent target); the aim in 2021 is for a 75 per cent online response rate.

Canada, which carries out censuses every five years, has successfully collected online responses since 2006 (with 2016 hailed as its “best census since 1666”). Yet New Zealand’s first online census last year spectacularly unravelled, with one in seven failing to complete it, and protests over the omission of the Pākehā ethnic category and the lack of gender options outside the male/female binary.

There has also been a row in the US over Donald Trump’s abortive attempt to add a question about citizenship to the 2020 census questionnaire. The question “Is this person a citizen of the United States?” has not appeared on a US census form for the entire population since 1950. The Supreme Court blocked its inclusion this year, and critics called Trump’s attempt politically motivated and warned it would put immigrant households off complying, for fear of their data being used against them.

Although the UK has a history of census compliance, there are always difficulties. The 25,000 “lost” people in Manchester and the 100,000 people missing from the Westminster figures according to the statistics watchdog in 2001, for example. Or the “conscientious objectors” convicted for their refusal to fill in the 2011 census because of the involvement of defence contractor Lockheed Martin UK.

There was a sharp rise in the rate of prosecutions in 2011 – with 157 prosecutions brought to court, resulting in 120 convictions (an increase on 2001’s 43 prosecutions, resulting in 38 convictions). Those found guilty face a maximum fine of £1,000 and a criminal record, but individuals are given the opportunity to complete the census form in court and be let off as a “conversion”.

Boycotts have been staged at times of political turmoil. The suffragettes organised a boycott of the census in 1911, because “If women don’t count, neither shall they be counted”, according to one of the placards. Women spoiled their forms, or spent the night away from home (as the householder had to list everyone who spent census night on the property). They organised all-night parties, events and concerts to attend, sat in cafes and restaurants that opened late, stayed out walking together throughout the night or simply hid (some in “barns and hay lofts”, and one in a bike shed behind her house). A large house in Manchester was renamed “Census Lodge” because it was so packed with “census slinkers” that night.

Once more, in 1991, a “missing million” people dodged the census due to the Poll Tax debate at the time. Reportedly, they feared they would be registered for the loathed tax – which had led to riots the year before – if they filled in the form, and wanted to avoid it.

At such a turbulent moment for British politics, could these past dramas be repeated? Brexit has already thrown a spanner in the spreadsheet. The bill that puts new (voluntary) census questions about sexual orientation and gender identity into law was dropped when parliament was prorogued last month. It had its second reading in the House of Commons in July, but since the hiatus, a date still has yet to be announced for its next stage, which is the committee stage in the Commons. This is significant because it leaves the biggest changes to the next census in limbo. (Another addition is the first-ever question about serving in the military, but it is not part of the prorogued legislation.)

“It’s a big groan just at the point when that one was going to be dealt with,” says professor David Martin, deputy director of the UK Data Service, and coordinator of the Economic and Social Research Council Census Programme from 2002-12.

“We’re still a reasonable way off [the next census], so it’s not an operational nightmare yet,” he adds. “It’s just messing around with priorities and timescales and things for the poor people at ONS who have got to do this stuff.”

An ONS spokesperson tells me the body is continuing to “work closely with the Cabinet Office to ensure all necessary legislation is in place”. Yet there is also the fear that the current climate of misinformation and suspicion of experts could lead to less trust in the census next time round.

“It’s been an issue in all modern censuses,” says Martin, who is in his fifth decade of working with census data. “Census agencies have got to work harder and harder in order to stand still… We do know from all of the kind of behaviour work that the ONS does, there are people who are suspicious of giving their data to government anyway, and that that pressure generally is increasing.”

He points out that in the case of censuses and official surveys globally, “generally speaking you see a pretty steady decline in responses to official surveys and other comparable things around the developed world. It’s going to impact on this census in the UK particularly because of all the other stuff that’s going on at the moment…

“That’s just a reflection of a broader change in society, but it’s certainly a factor. And nobody really knows how that plays into the [preferred] online [policy] and whether the two interact, because we’ve not seriously tried it that way before. It’s pretty hard to read how the current political climate might impact on people’s inclination to engage,” he adds. “And ONS recognise that, but obviously can’t do much about something that’s going to happen in spring 2021 at the moment apart from making sure they’re really nimble with the PR around the census when we get much closer.”

Speaking to me in a personal capacity, Dr Oliver Duke-Williams – a member of the Methodological Assurance Panel, which advises the National Statistician on future censuses beyond 2021 – echoes these concerns. He is an associate professor at University College London in the Department of Information Studies, and has long experience of working with and disseminating census data.

“There is growing distrust of authority perhaps, or certainly we’re told that there is, and you can look to the ‘had enough of experts’ sort of view – distrust of, I guess, people like me, who try to encourage people to fill in their census form,” he acknowledges.

“So yeah, it is a concern. And that plays into the worries about what happened in New Zealand last year. There’s always a risk of boycotts and campaigns against the census. They’ve always happened.”

Trust in statistics produced by the ONS has remained high (85 per cent) and stable since 2016. But the main reason behind lack of trust is the belief that government and media misuse the statistics, according to the National Centre for Social Research (NatCen).

“Obviously people have legitimate concerns about filling it [the census] in, and there is lots you can find in the press or online, reputable sources as well, about use of officially-gathered data for immigration checks and that sort of thing, and accusations of overreach by the Home Office in use of data from schools, national pupil database, hospital records,” says Dr Duke-Williams.

“So I fully understand why people are concerned about putting their trust in government collecting of data. There’s always a need to convince the population, convince people filling in the census, that census agencies are reliable and responsible.”

An ONS spokesperson says: “At the heart of the census is keeping information safe, confidential, secure and private – no one can find out an individual’s details for 100 years – and that is made clear to everyone taking part in the census. ONS is an independent body. We are actively working with other partners and have a network of community engagement officers who work in local communities to build trust so that everyone will be included in the next census in 2021.”

Another consideration of a digital-first census is ensuring people can access the form. Paper forms will still be an option, and sent out to certain areas “where they suspect online response will be low”, says Dr Duke-Williams.

“We know there are going to be problems filling it in online for some people. So: who isn’t able to use the internet? Who can’t fill it in online? You can look at filling it in at your local library and having sufficient support there, or other community locations where you can offer people service when the problem is lack of broadband provision, or things like that.”

The ONS is focusing on communication and outreach plans as well as learning lessons from the rehearsal areas. For example, in east London, there are community advisers working to engage Bangladeshi and Somali communities in the census process.

For all the new approaches and modern concerns, however, “the motivation for actually collecting census in the first place remains fundamentally unchanged”, says Professor Martin. “In the sense that it is the single most comprehensive attempt at getting population characteristics which we undertake.”