Hidden populations

We’re just home from a long day filming at the beach. Aside from a seagull-poop incident and several drippy ice creams, the day was a great success. One of our main aims was to interview Ruth about her research as Thomas Bayes’ Chair of Statistics.

Ruth uses what’s called the capture-recapture method to estimate the sizes of populations. As an example, we looked at a population of rubber ducks in a paddling pool. How many were there? Well in practice, we could have just counted them, but with real life ducks that wouldn’t be so easy, as they might swim around, and we might not be able to see them all at once. This is where the capture-recapture method comes in handy.


Assisted by Ruth’s son Alex, we took a sample of ducks from the paddling pool, and marked them with crosses, before replacing them in the pool. We then took a second sample of ducks. By comparing the number of ducks that were seen both times, only the first time, or only the second time, we can estimate the number that we didn’t see at all, and hence the total number of ducks.

Alex Ducks.png

This simple method is used in practice to estimate the size of so-called ‘hidden populations’ – populations that can’t easily be counted. Such populations could be groups of animals, but they could also be modern-day slaves, illegal immigrants, or tax-evaders.

Ruth explains a lot more about the capture-recapture method and how she uses it in her research in Week 3 of the course. She was also invited to speak about the method by the London Mathematical Society as part of their 2015 Public Lecture series, and you can watch the video here.


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