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Counting pictures

By David, 19 June 2015 Activity

A picture of the cover of Double Helix magazine.

Choose a picture in your magazine at random.

Want to practice how scientists count whales in the wild? In this activity, we’ll use mark and recapture to count the pictures in a magazine!

You will need

  • A magazine
  • Pen and paper
  • Two smartphones or cameras (optional)
  • A friend to count with

    Somone taking a photo of a magazine.

    Take a photo of your picture.

What to do

  1. Open the magazine to a random page, and randomly pick one of the pictures on that page. Take a photo of it, or write a short description. Then, close the magazine.
  2. Repeat this process 19 more times, so you end up with 20 samples.
  3. Pass the magazine to your friend, and ask them to follow the same process. To make it a bit faster, only get them to record 10 samples.

    Two phones, each displaying photos of the same thing.

    Compare your sample to the one your friend took.

  4. Compare the two sets of samples. Were there any pictures that you both photographed?

The calculation

  1. First, count how many of your friend’s samples were the same as yours. Then calculate it as a percentage. This is an estimate of how much of the magazine you sampled. The calculation multiplies the number of pictures that are the same by 10 (the number of samples taken by your friend):

Percentage you sampled = 100% x Number of pics the same ÷ 10

  1. To convert your answer into a population estimate (how many photos are in the magazine) you need to do another calculation. Multiply it by 20 (the number of samples you took):

Population = 20 x 100% ÷ percentage you sampled

What’s happening?


Follow the calculations to estimate the number of pictures in your magazine.

This is a technique known as mark and recapture, which is often used to count animals in the wild. To conduct a mark and recapture survey, an expedition catches several individual and marks them in some way. This could involve banding, tagging, or taking genetic samples.

A second expedition then visits the same area and checks each animal they catch to see if it was marked. By comparing marked and unmarked animals in the second expedition, scientists can calculate the percentage of the whole population that is tagged. This measurement can be combined with the total number of tagged animals to make an estimate of the whole population.

This technique assumes that each animal is equally easy to catch. If some are easier to catch than others, it can distort the final estimate. In this activity, big pictures may get picked more than small ones. This means your estimate might be less than the real total.

More information

Using mark and recapture to count whales

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