What is the capture-recapture method?
- This method is used when you want to estimate the size of a population
- It is used when it is impossible or time-consuming to count the whole population
How do I use the capture-recapture method?
- Let N be the size of the population
- Take a first sample (the capture) of size M
- Tag them with a mark or label
- Release them back into the population
- Give them some time to mix with the rest of the population
- Take a second sample (the recapture) of size n
- Let m be the number of them that have been recaptured (previously been tagged)
- Form an equation by assuming that the proportion of the second sample that have been tagged is equal to the proportion of the population that has been tagged
- In words this is
- Rearrange this equation to find an estimate for the population size N
Roger captures 50 rabbits from Lopital Woods and marks them by putting a safe tag on their ears. Roger then releases the rabbits back into the woods. One week later, Roger captures 100 rabbits and finds that 8 of the rabbits have the tag in their ears.
Use the capture-recapture method to estimate the size of the population of rabbits in Lopital Woods.
Let N be the size of the population.
Write down the fraction of the population that have the tag:
Write down the fraction of the second sample that have the tag:
Form an equation by making the two fractions equal:
Divide both sides by 8:
Write your answer: There are aound 625 rabbits in Lopital Woods