Net Catch Statistics In Your World 
Student Notes
Teachers Notes
Recapture 20
 
Recapture 40
 
Capture 40, Recapture 20
 
Capture 40, Recapture 40
 

How Many Beads?
You are going to use the capture-recapture method to estimate the number of beads in a container. By repeating the method you will get different estimates for the same total. You will find out how accurate your estimates are and see the effect of sample sizes.

You will work in small groups.

Recapture 20
You will need:
A container of beads all of one colour, 20 beads of a different colour, Table 6 on page R1.

  1. Remove 20 beads from the container and replace them with the 20 beads of a different colour. These are the 20 'marked' beads in the container. Mix up all the beads in the container.
  2. Take a sample of 20 beads from the conta'mer. Count the number of marked beads. Use this to estimate the number of beads in the container. Enter the number of marked beads and your estimate in Table 6. Return the beads to the container and mix in well.
  3. Repeat b nine more times so that you have 10 estimates of the total in Table 6.
  4. Add the 10 estimates in Table 6 and divide by 10 to find the mean (average) estimate.
  5. Count the number of beads in your container. Were any of your estimates accurate? How accurate was the mean of your estimates?

 

Recapture 40
You will need:
A container of beads all of one colour, 20 beads of a different colour, Table 7 on page R2.

  1. Remove 20 beads from the container and replace them with the 20 beads of a different colour. These are the 20 'marked' beads in the container. Mix up all the beads in the container.
  2. Take a sample of 40 beads from the container. Count the number of marked beads. Use this to estimate the number of beads in the container. Enter the number of marked beads and your estimate in Table 7. Return the beads to the
    container and mix in well.
  3. Repeat b nine more times so that you have 10 estimates of the total in Table 7.
  4. Add the 10 estimates in Table 7 and divide by 10 to find the mean estimate.
  5. Count the number of beads in your container. Were any of your estimates accurate? How accurate was the mean of your estimates?
  6. Compare your estimates here with those in Cl. Which were in general more accurate?
  7. How does the size of the recapture sample affect the accuracy of your answers?

 

*Capture 40, Recapture 20
You will need:
A container of beads all of one colour, 40 beads of a different colour.

  1. Remove 40 beads from the container and replace them with the 40 beads of a different colour.
  2. Take a sample of 20 beads. Use this sample to estimate the number of beads in the container. Return the beads and mix well. Repeat until you have 10 estimates.
  3. Compare your estimates with those in Cl. How does the size of capture sample affect the accuracy of your estimates?

 

*Capture 40, Recapture 40
You will need:
A container of beads all of one colour, 40 beads of a different colour.

  1. Remove 40 beads from the container and replace them with the 40 beads of a different colour.
  2. Take a sample of 40 beads. Use this sample to estimate the number of beads in the container. Return the beads and mix well. Repeat until you have 10 estimates.
  3. Compare your estimates with those in C2. How does the size of capture sample affect the accuracy of your estimates?

 

Back