Net Catch Statistics In Your World 
Student Notes
Teachers Notes
A Practical Example
 
Practical Problems
 

The Practical Approach

*A Practical Example
If there is an area in your school where a fairly large, but unknown, number of pupils go each day (the playground at break for example), you can try to estimate the number of pupils in your school using the capture-recapture method.

One day you will go to the area and 'mark' some pupils.

On the next day a different person will go to the same area and interview some pupils to find who was marked' on the first day.

  1. Discuss the following points:
  2. How, are you going to mark pupils?
  3. How, will you choose pupils to mark?
  4. How many pupils will you mark?
  5. How many pupils will you interview on the second day?
  6. What will you ask them?
  7. What other practical difficulties will.vou have to solve?

Have a final discussion with your teacher.

  1. Do the experiment, record your results and work out your estimates.
  2. What problems did you find with this experiment?

 

Practical Problems
When we were looking at how many fish there were in a pond in Section B, we made some assumptions.

We assumed that:

The same fish were in the pond when we took the capture sample and recapture sample.

The marked fish mixed completely with the other fish between the two samples.

Each individual fish was equally likely to be caught at the recapture stage.

  1. Can you think of any other assumptions we have made?

Look at the questions in the introduction about counting whales, cod and herring and the practical example in D1. For any one of these, answer questions h to d.

  1. How are these assumptions likely to be wrong?
  2. How do you think this will affect the estimates?
  3. Can you think of any other problems which would make the capture-recapture method difficult to carry out?
  4. Can you think of any other problems the capture-recapture method could solve?

 

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