I believe statistician ecologists use the reverse in an interesting way. Tag 100 units from a species, say goats (failure of imagination on my part) from a cert

I believe statistician ecologists use the reverse in an interesting way. Tag 100 units from a species, say goats (failure of imagination on my part) from a certain location. Then sample 100 other animals at a different time, controlling for time spent and so on, and the ratio of tagged animals that come up in the result will give statistical information about the total population of said species.