Amid endless flyers from food delivery companies dropping through the letterbox, households in England and Wales will soon receive a postcard from the Office for National Statistics (ONS) giving notice of the 2021 census. It will be followed by a letter advising that every household must complete a detailed questionnaire about themselves and their co-residents on 21 March. The letter will contain a unique code enabling recipients to complete the census forms online, although paper forms will be available on request. In the weeks following census day, 30,000 field officers will knock on the doors of those households that have failed to return a completed questionnaire. The officials will begin with explanations and encouragement, but the process can end with a court appearance and a £1,000 fine. The once-a-decade exercise to take a snapshot of the country in one 24-hour period is a major challenge. This time it is complicated by the Covid-19 pandemic, but the ONS is confident there will be high levels of compliance and the vast majority of households will answer questions online. Some pockets of the population are hard to reach, for reasons that include language barriers, mistrust of government, worries about personal data and aversion to or difficulty with form-filling. The overall response in 2011 was 94%, but among some BAME households it was up to 10 percentage points lower. Hundreds of community engagement advisers have been recruited to work with different ethnic and religious groups this time. Adel Khaireh of the Greenwich Islamic Centre in south-east London is leading efforts to engage the Muslim community. “Trusted members within our community are appearing in videos which can be shared on social media,” he said. “The imam of the mosque will be explaining how important it is and reminding people to fill in the form. People will think: ‘If the imam trusts this, then so should we.’” The data gathered from the census would not only help the government to plan public services and housing needs for the next decade, but would provide the mosque with a better understanding of its community, he said. The last census documented 17,349 Muslims in the borough, 6.8% of the population, but Khaireh believes the figure today is closer to 30,000. “We want to measure our growth as a community, plan for how we can serve the community and address specific groups within the community, such as Somalis,” he said. Emily Stidston, the ONS’s census community partnerships manager, said one area of concern was how personal data would be used. LGBTQ+ people in particular may be worried about new questions on sexual orientation and gender identity, and migrant or refugee communities may be concerned about data being shared with the Home Office. “No one can be identified from the census data, everyone is completely safe filling out their census. All the ONS wants are hard, anonymous facts,” she said. Mistrust of government was a core concern among some population groups, she said. “In other cases, people have simple misunderstandings foremost in their mind. For example, I have had to reassure people that filling out the census will not lead to doorstep marketing for double glazing.” The census form asks questions about the makeup of households, type of accommodation, occupation, education, ethnicity, religion, marital status, state of health and means of transport to work. In 2011, 270 people were found guilty of non-compliance and ordered to pay a fine averaging £280. The ONS estimates it will take a year to process the data. As well as routine government planning, information from the 2021 census will help preparations for possible future pandemics and other emergencies. Northern Ireland will also hold a census on 21 March, but Scotland has postponed its census until 2022 because of the pandemic.