Censuses are the oldest official statistic produced by Statistics Netherlands (CBS). They are a treasure trove of demographic and socioeconomic information. CBS organised a census at the request of the European statistical office Eurostat. A census is held once every 10 years. However, people living in the Netherlands notice little of this process, because CBS has been organising censuses digitally for over 40 years. Meanwhile, the first results of the 2021 census are known and have been delivered to Eurostat. CBS is now working hard on the second part, to be completed in March 2024.
In many countries, a census is a huge operation, with hundreds of thousands of interviewers who go door to door with a questionnaire to ask the inhabitants about their family composition, level of education, occupation, dwelling, etc. But in the Netherlands, the census is carried out in a completely different fashion: digitally. This is because the Netherlands has a good municipal Personal Records Database (BRP), which CBS is permitted to use under the Statistics Netherlands Act. Another important source for data is the System of Social Statistical Databases (SSB). ‘As a result, for more than four decades the inhabitants of our country have not had to fill out census questionnaires. This lowers the survey burden while it also saves our country millions of euros in comparison with other European countries, where census takers still go from door to door,’ explains Reinoud Stoel. He heads the methodology team that has long contributed to the delivery of census results at CBS in The Hague.
Initial results of the 2021 census
As a researcher at CBS, Eric Schulte Nordholt has been closely involved in the national census for more than two and a half decades. He knows all the ins and outs, and is frequently consulted as an expert in this area at international level. In addition, he participates in various European task forces on the census, and is the international coordinator for this topic at CBS. ‘We were able to deliver the first results to Eurostat at the end of last year. Previously, the data had to be delivered per country at the municipal and regional level. This time, Eurostat required us to deliver data at ‘grid square level’ on top of it: 42 000 squares in total for the Netherlands. All of that comes with metadata, to be described in a few hundred pages. That’s quite a job!’
The count at grid level is new in the current census and offers much more detailed information than at municipal level, Schulte Nordholt explains. ‘The grid of 1 by 1 kilometre squares cuts through municipal borders, but it also needs to match our national borders. One positive change is that the grid remains stable over time, unlike municipal borders.’ Eurostat needs the grid data in order to distribute funds among the member states in support of economic development, the so-called structural funds. In addition, the information will be necessary in the years ahead to be able to make more detailed comparisons within Europe.’
Information security plays a crucial role in the delivery of grid square data. Marieke de Vries is a methodologist at CBS and closely involved in that process. ‘CBS has worked with several other statistical offices to develop two new statistical disclosure control (SDC) methods and the accompanying software, allowing secure publication of the grid data. This means no information about separate individuals can be retrieved. We apply this principle in all our official statistics. It is done in such a way that the published results do not differ much from the actual results.’
New SDC methods
According to De Vries, the implementation of SDC methods in the Dutch census is nothing new: ‘With the previous method, a lot of detail was lost due to the security. That is why we developed new methods in which more detail can be published while still protecting the privacy of Dutch citizens. These new methods are complicated, so they are explained in a background ‘Statistical Trends’ article (Dutch only). Information security is a top priority for CBS: data are safe with us.’
At the moment, CBS is working on the second part of the census data. That part must be completed and delivered to Eurostat by March 2024. Schulte Nordholt explains: ‘The second part consists of so-called hypercubes. These are high dimensional tables that store, for example, data on the population, people in work and dwellings at the municipal level. We have all such data in house and they needed to be validated. We only had to estimate data on occupations.’ The March 2024 deadline is well within reach for CBS and a number of other European countries that conduct censuses based on registers. Stoel adds: ‘We have a lot of people from different departments of CBS working on this census, and the collaboration is excellent. It’s an achievement of which we are proud.’