Estimating the prostitution population using online data: a single-registration approach for the Netherlands and Belgium

Evidence-based risk-prevention policies for people involved in prostitution require, amongst others, reliable population estimates. This study proposes a methodological framework and novel data source for measuring their population in regions where the internet plays a predominant role in the industry. We use online data that documents activities in prostitution markets in the Netherlands and Belgium. We derive population estimates using the single-registration Zelterman approach—a variant of single-source capture-recapture techniques—which models unobserved sex workers based on the counts of observed sex workers. The combination of this novel data source and method allows us to estimate the number of sex workers active between February 2019 and 2020 in the Netherlands and Belgium. These resulting estimates for the Netherlands and Belgium are lower than previously published estimates. We find that, relative to the overall population of the two countries, the proportion of sex workers is roughly identical despite differing legal environments.