9 October 2016

New GCSE: Capture-Recapture

If your school plans to use Edexcel for GCSE from 2017 then you may have spotted the capture-recapture method in the Higher Tier specification:
Source: Edexcel GCSE (9 - 1) Mathematics Teaching Guidance

Here's the sampling content in more detail:

Source: Edexcel Content Exemplification FAQs

It's worth noting that both Edexcel and AQA list stratified sampling as a topic that has been removed from GCSE. However, this comes with a caveat - both sampling and proportional reasoning do feature in the 9 - 1 GCSE, so it would be reasonable for exams to include a stratified sampling question even if students haven't been explicitly taught this topic (as long as the question doesn't use the word 'stratified' without explaining its meaning). So my advice - whatever board you're using - is to look at a few stratified sampling questions with your GCSE class, whether in a statistics lesson or a ratio and proportion lesson.

Stratified sampling is a great opportunity to use proportional reasoning, as is the capture-recapture method. If you've taught Edexcel's GCSE Statistics then you'll already be familiar with capture-recapture, but I'll explain it here in case you've not seen it before.

A simple example
Try this question... it will only take you a second.
I captured 50 fish from a lake. I marked a big cross on the back of each fish with a permanent marker*... 
I put the marked fish back in the lake and they happily swam away to join their friends. 
The next day I captured 20 fish from the same lake. 10 of them had a cross on their back. 
Can you estimate the total population of fish in the lake?
*no fish were hurt, promise.

I'm sure you spotted that the proportion of marked fish in the second sample was 0.5, and we can assume the same proportion of marked fish in the whole population. Given that I marked 50 fish, we can estimate that there are 100 fish in the lake.

A formula
If the numbers are less straightforward so the estimation can't be done mentally, it's easy to set up a formula to work out the population. This is certainly not a formula that students will need to memorise - it can be deduced using proportional reasoning.
You can see that the formula on the left simply shows that proportion of marked fish in the population is equal to the proportion of marked fish in the sample. The formula on the right has been rearranged to make N the subject.

Here's an example from the Biology section of BBC Bitesize. It would be better if they had shown the proportional reasoning and rearrangement process rather than just give a final formula.
Source: BBC Bitesize

The example goes on to list some assumptions - these are certainly worth discussing with your students.
  • There is no death, immigration or emigration (ie the population is closed)
  • The sampling methods used are identical
  • The marking has not affected the survival rate of the animals 

We also assume that animals do not lose their marks, that marking does not affect the likelihood of recapture, and that sufficient time is left between marking and recapture for all marked individuals to be randomly dispersed throughout the population.

An exam question
So what might this topic look like in a GCSE exam? Here's a question from Edexcel's 2014 Higher GCSE Statistics paper.
A scientist wants to estimate the number of fish in a disused canal. He catches a sample of 30 fish from the canal. He marks each fish with a dye and then puts them back in the canal. The next day the scientist catches 20 fish from the canal. He finds that 4 of them are marked with the dye. 
(a) Estimate the total number of fish in the canal. (2)
(b) Write down any assumptions you made. (2)

For part b, candidates have to mention two ideas, including something about the population being unchanged, or the idea of randonmess, or that markings remain unchanged.

For more exam questions, visit Edexcel's Emporium and look under GCSE 1MA1 Practice Papers > Themed Papers.

Teaching ideas
I think this will be quite a nice topic to teach. Here are a few useful links:

See my data resource library for listings.

Finally, here's a nice video to show in your lesson - Johnny Ball estimates the number of black cabs in London.


  1. The Johnny Ball video is lovely, but I don't think I like the way round he did the calculations. I prefer to work with (using his ping pong numbers) 17/100=0.17 or 17%, then question 100=17% of ?. I find this way tends to make more sense conceptually to students (although, granted, I've not taught this topic to huge numbers of children).

  2. I agree with jemmaths. He also wrote an equals sign connecting two calculations that were not equal...

    1. I agree too. Don't show it then! I'm keeping it in the post because I think it's worth maths teachers watching it - I like the context he uses.

  3. There are a number of resources for this topic here: