Residents on Anglesey have taken part in a rehearsal for the next census, including filling in the questionnaire online for the first time.
More than 33,000 households were asked to answer 57 questions on Sunday in a test run of the 2011 census.
Along with the option of filling in the questionnaire on the internet, they will also be the first to be able to mark their identity as Welsh.
The rehearsal is seen as important to ensure the real thing runs smoothly.
The results of the population survey are used by the UK government to help allocate money to areas to spend on services such as transport, housing, health and schools.
The 2001 census was marred by rows when Scottish people, for instance, could tick themselves as "Scottish", but "Welsh" was absent from the form.
The Welsh tick box option is proposed for the next census, planned for March 2011, as part of a suite of questions about nationality, ethnic group and language.
The Office for National Statistics (ONS) is responsible for carrying out the census in England and Wales and a test run will take place at three locations in Wales and England: Anglesey, Lancaster and the London borough of Newham.
Earlier, Anglesey AM Ieuan Wyn Jones urged all residents to take part.
"By taking part in this rehearsal, the residents of Anglesey will play an important role in helping ONS make sure that everything will run smoothly in the 2011 census, including testing the new online questionnaire," he said.
"The rehearsal is fully bilingual and I urge everyone to make it a success - and play our part in helping the 2011 census shape future funding and services."
Questionnaires in Wales traditionally have one extra question than those in England, asking people about their use of the Welsh language.
The questions proposed by ONS for the census have to be approved by Parliament before they are included on the 2011 document.
For the first time, it is proposed people who consider themselves Welsh will be able to tick "Welsh" as their national identity, even if they live in England.
For the 2001 census, campaigners said the absence of a tick box for Welsh identity was an "insult" to Wales.
They pressed the UK government to include a Welsh tick box on the forms, with a 10,000-name petition handed to then Prime Minister Tony Blair.
• The Northern Ireland Statistics and Research Agency (NISRA) is responsible for conducting the census in Northern Ireland and it has a census rehearsal in two areas: Derriaghy and Moy and Benburb in October.
• In Scotland the General Register Office for Scotland (GROS) carries out the census. A rehearsal took place in March in west Edinburgh and Lewis and Harris.