The Capture-Recapture Method to Estimate Class Size

Written by Daniel T. Bobrowski, University of North Carolina at Chapel Hill

Learning Outcomes:

- To solidify student understanding of the capture-recapture method by kinesthetic reinforcement

- To reinforce the idea that the capture-recapture method provides only an estimation of the total population size

Activity Description: Student volunteers will apply the capture-recapture method to a classroom filled with their peers. (This is an activity that works well in large classes.) The data collected in this procedure will then be applied to the Lincoln-Petersen equation to determine an approximate total population size for the class.

Time Needed: 15 minutes

Materials Needed: Student volunteers and squares of paper as “tags”

Activity Instructions:

  1. Give a preliminary description of the capture-recapture method. Students should have read the pertinent textbook sections prior to this class meeting.
  2. Ask for two student volunteers to act as scientists attempting to determine the population size of college students in the lecture hall.
  3. Explain to the class that one student volunteer will be conducting the first sampling of the population by capturing individuals of the population, tagging them, and then releasing them back into the wild.
  4. Ask the class what some qualities of an ideal tag would be. (An ideal tag would not impede the normal life of the animal but would be easy for scientists to spot and identify.)
  5. Have student 1 walk around the classroom selecting classmates. A “tag,” or square of paper, is handed to each student that is considered captured.  While student 1 is conducting the sampling, student volunteer 2 should be outside the classroom to avoid sampling bias during the recapture procedure. An alternative procedure to having a student volunteer conduct this first sampling would be to have teaching assistants tape pieces of paper that read “trap” on the bottom of seats throughout the lecture hall.  The first sampling would then consist of notifying students that if they had sat in a “trap,” they were considered captured and tagged.
  6. Ask the class why some time should be allowed to pass between the first and the second samplings of the population. (Sufficient time must be allowed for the tagged individuals to redistribute themselves among the population.  This allows for the assumption that all individuals have the same probability of being captured in the second sampling.)
  7. Student 2 will then conduct the second sampling of the population by walking around the room and selecting students for capture. Students captured in this second sampling that are already tagged are considered recaptured students. Students that are captured for the first time are just factored into the total number of the second catch. The second sampling should be proportionally larger than the first one.
  8. Use the numbers from the demonstration to demonstrate on the whiteboard how to solve for the total population size in the Lincoln-Petersen equation:Where: M = The number of individuals caught and marked in the first sampling

    N = The estimate of the total population size

    R = The number of tagged animals that were recaptured

    C = The total number of individuals captured in the second sampling

  9. While the capture-recapture demonstration is taking place, have the teaching assistants take a precise head count of students in attendance (or take attendance via a clicker system). Compare this number to the experimentally determined figure. Use this discrepancy to illustrate the fact that this method provides an estimation of the total population size.
  10. Ask the students why the Lincoln-Petersen method is only an approximation of the total population size. (Realistically, not all individuals have the same chance of being caught in the second sampling because of the possibility of a learning curve, death, etc. Also, stochasticity or chance could factor into and distort the final result.)

Possible discussion/test questions:

  1. Suppose 40 fish in a pond are captured and tagged. A week later, 100 are caught, and of these, 5 have tags. What is the estimation of the population? N = 4000 / 5 = 800
  2. What would happen to our estimation if organisms with tags made the recapture easier? Would the estimate be too high or too low? The estimation would be too low. For the example above, the recaptured would be more than 5, making N = a lower number.

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