The population size is a calculation of the number of people engaged contextually on a topic. It includes people both actively and passively (even unknowingly) engaged around a topic.
Population is calculated simply by modelling the meta-data collected at the top level.
When you run a search, we know the topics that sit within the contextual universe of that searched topic, and we have the related demographics, and we model this out to understand the size of the population, as well as the demographics of the population engaged within the context of the search term.
