Lesson video

Hello, my name is Miss Parnham.

In this lesson, we're going to learn about Capture-recapture.

Capture-recapture is a way of estimating the number in a population.

Often of things like wild animals.

Let's take a look at this example.

So one day a farmer catches 90 mice, tags them and releases them.

The next day, she catches 50 mice and finds that 15 are tagged.

So the information that we glean from this is that 15/50 or 3/10 of 30% of the mice are tagged.

using that information, we say that 30% of the population must be tagged, So if 30% represents 90 mice, then 10% is 30 mice and 100% is 300 mice.

Now, in this example, we assume that the mouse population hasn't changed from one day to the next.

So no mice have been born, and no mice have perished, in between the capture and the recapture.

Here's a question for you to try.

Pause the video to complete the task, then restart the video when you're finished.

Here are the answers.

So in this first question, we knew that 1/6 were tagged, so we assume 40 is 1/6 of the population.

And then multiply 40 by six to estimate that population.

Here is another question for you to try.

Pause the video to complete the task and restart the video when you're finished.

Here are the answers.

The assumption we have to make about the snails is that none have died and none have been born in the 24 hours in between the capture and recapture.

So this is part of the reason it can only ever be an estimate for a population when we use these methods.

Here is a further question for you to try.

Pause the video to complete the task and restart the video when you're finished.

Here are the answers.

This debt is probably slightly more reliable than the capture-recapture of living things.

Because we can be fairly sure the population of buttons has stayed constant in between Dexter's capture and recapture sessions.

This question is slightly different.

A gardener catches some caterpillars marks and releases them.

And then the next day she catches 28 caterpillars and finds that four are marked.

From this data, she estimates that she asked 210 caterpillars in her garden.

So how many caterpillars did she catch the first time? We'll start in the same way by predicting what fraction of a percentage of the population of caterpillars attacked.

We do that using 4/28 or 1/7.

So if we assume that 1/7 attacked, so she's estimating that 210 caterpillars are in her garden, then 1/7 of this 210 will be tagged.

So she must have caught 30 caterpillars on the first day.

Here's a question for you to try.

Pause the video to complete the task and restart the video when you're finished.

Here are the answers.

This question is looking for the number that Eva initially catches so that's not an estimate.

So we find the tagged rabbits are estimated to be 3/8 of the population.

So, if she used her figures to estimate the population at 120, then 3/8 of 120 is 45 rabbits were initially caught.

That's all for this lesson.

Thank you for watching.